I love birthdays parties near Halloween! My sister is 13 years younger than me and I always used to beg my Mom to let me plan her parties! Now I am the Mom so I'm pretty big with keeping everything in and on a budget. It's great that you will already have the kids dressed up. So that will take care of the number 1 step of making sure they are in costume. So here is what I would do:
Decor-Boas to decorate as streamers or to place on tables, easy to find at a dollar store. I'd have lots of white mini lights (since you wouldn't want to use too many candles around the kids). Buy some dollar store mardis Gras Masks and beads. You can use these on tables or to pass out to the kids. Another dollar store find. There is a gell like candle that can be bought at some dollar stores. Lookes like solidified jello...for areas out of the kids reach. Also, I'd use fabric linens with small mirrored pieces on the tables.Add colorful glass stones for color.(again dollar store. I'd also go to thrift stores such as Salvation Army to find some silver style dishware. Very cheap there,and the kids will think it's very 'fancy' that they are eating their food out of these dishes.
You can go with an Italian theme(where the masquerad balls originated) which would be more architectural based and more adult like(in my mind!) or go more with a New Orleans flare, which for me would be more fun for the kids. Sheer fabric in colour or white can be found at fabric stores or WalMarts,hang them near and around the mini lights or around tables. Black balloons (with purple even) would work great since it is so close to Halloween. You could also have someone dressed up as a jester..if someone you know is so bold. :) You can grab old wooden chairs and spray paint them gold. Pearl garland can be placed about and stuffed in and hanging out of glass vases.
Games/Crafts:Print out sheets for them to make their own masks here: http://parenting.leehansen.com/Printables/Masquerade/purple-specs.pdf You can cut them out prior to, or the kids can color them at the party then do the cutting. Buy some craft feathers,sparkles, glitter,etc to decorate them with. Same can go for with pipe cleaners and jewellery such as bracelets and necklaces. You can go with games such as the fishing game. Have a tub,bucket,small kids pool with a partition between. The child hold onto a mock fishing pole and hooks onto a prize,or a (dollar store) jewelled ring or bracelet. The one who gets the 'biggest jewel' wins. You can also do a game (if you have the space) of Clue. Somebody disappeared and there are clues set up to go hunt for this person.
Food: If you are going with the New Orleans theme,I'd do finger foods. Especially where the kids are younger. You can still serve them on the platters and such. So you could go from rolled cream cheese sandwiches, to fruit kabobs, to stuffed mushrooms, marinated olives, to a more southern style of food such as Shrimp rings, if the kids are into crab, then by all means crab! :) BBQ'd Pulled pork sandwiches, mini quiche. With the sandwich idea, it'd be sort of like a tea party..assortment of sandwiches.(which is the easiest, kids at that age aren't too picky about what food they will be eating. And you can get creative with the assortment of sandwiches)
You can also have a chocolate fountain and fondue if you want to go all out. Im sure those can now be bought in department stores, or can be rented at your local party supply store.
I'd definitely do a punch. There are so many variations,my favorite usually contains frozen pink lemonade, bottle of gingerale, bottle of Sprite, and a can of frozen Cranberry or Orange Juice. You can add frozen strawberries.
Cake:You can always get one professionally done. They aren't cheap, but that is up to you. Yes, I am cheap. I go to my local grocery store and get a nice big $20 double layered pound cake (no flowers) and either get them to decorate with icing which would cost more, or I decorate it myself. Michaels (craft store) has great assortment of cake supplies. Such as edible glitter. If you google Masquerade cake, lots of pictures come up. Just depends on what you want to do,make it yourself or get one professionally done. Also, you can do individual cupcakes.

I hope this has helped, feel free to throw more questions my way. I love planning and preparing, it's a lot of fun for me. I hope the party is a huge success! :)
Take care!

Pin the tail on the donkey from Shrek. Pumpkin and ghost shaped sugar cookies. Draw faces with magic markers on pumpkins. Pumpkin pie. Cut out ghosts from scrap printer paper and hang them around the room on string. Find and dance to, "The Monster Mash," for music. If they get tired, put a lot of blankets on the floor and show "Casper." They might nap.

I think you have do some of following steps :

1) Make a good Chocolate cake.
2) Decorate room with some cartoon pictures,balloons,some fancy stars.
3) you may try children's games like lemon race,singing chairs,dancing.
4) you also arrange mickey mouse character for children's

I have two kids, 4 and 8, who love theme parties. A few of the best parties we've thrown involved them dressing up in costumes (one was a Western theme and the other a Princess/Prince party). Kids love to dress up and pretend to be someone/something else.

My suggestion would be to go with simple costumes so no children will be left out due to low funds. One way to do that would be to simply ask the guests to dress in something nice. Then make the mask decorating a craft project at the party. Not only will every kid have one, it will help fill up some of the time.

How about making cute invitations that look like a Mardi Gras/Masquerade Ball mask?

Plan the party after lunch to cut down on food costs and serve fun finger foods and cake. Have someone serve them to kids on big metal platters to give the feel of a masquerade ball.

Decorations for a masquerade/costume party theme should be easy to find this close to Halloween. Or pull out the white Christmas lights early and hang them around the room to add some party lights (maybe even a silver disco ball).

Since the child and guests are still fairly young, don't kill yourself on planning in depth games. They'll be just as happy if you give them a bunch of balloons to bat around the room (again, the disco ball would be great for this!). Or play freeze dance by having them dance to their favorite songs and when the music stops, they must freeze.

If you have a week or more before the party, you can find some great, inexpensive party supplies at Oriental Trading. Don't forget to check out your local dollar stores for great prizes and decorations. It'll save you a ton of money.

I have four children aged 20 to 5(almost 6) and have done day care for 11 years. Masquerade parties are amoung their favorites. I don't know when your childs birthday is but a great way to have an interactive invitation is to send a "packet" with a cutout for a mask which they can decorate with things from home or you can add a few special items in each as well, these can be age/gender appropriate as well and do not have to be the same. Along with a self addressed pre postaged postcard for the RSVP. OR, you could have the kids make their own masks as a first activity to the party. Sequines, feathers, stickers, anything with "bling" kids love! Glue sticks and nothing too heavy are easiest and least messy for this if you use it as an activity. I would use poster board, precut the masks and have everything set out and be sure that each guest has a space at a table to do this.Consider if you will have helping hands at the party and/or how long you plan on the party last.
Creating a masquerade enviroment can be simple and cheap. Bright and colorful plates, tableware, tablecloths,napkins, etc can be purchased from any dollar store. Consider having a gift table, grazing table for finger foods and punch bowl (be sure this is set low so they can help themselves kids love that!) Let the food be easy, less mess for you, easy for the kids and able to focus on your child and interact during the party. Each table should/could have a center piece. One of these of course can be the cake. The cake is a good center piece. With this one perhaps just buying your childs fav cake and making a mask as a family to put on it would be nice. These can just be bright vases of flowers or you can use large bowls and fill each one with mardi grah beads (dollar store), liehs(dollar store), etc for the kids to take home in a grab bag. Mini lights on these tables would nice too. This next idea is cheap... a lot of work... but has a great wow affect and can be done well ahead of time. Kids love things that hang from the ceiling, I have used gift wrap ribbon, sat on the couch in front of the tv and made tons of curly cues zipping the ribbon between my thumb and one side of the scissors using different lengths, the night befor the party hanging them all over... another cool thing(if this party is in the evening) is after you hang them is to dab some glow in the dark paint just on the ends of them... turning the lights off is very cool! You feel like your in space! I really liked doing this because they can be left up for a period of time, allowing your child the "euphoria" memories of the party and it's just a fun thing to do.
Games: Only two or three and activities should be planned. They are young and you'll find that between the mask activity (if you choose to do that) eating, opening gifts and cake and ice cream the time seems to fly. Treasure hunts are the best! Since they are so young I have found that hiding little prizes to add to their bags is best, everyone wins. At that age, I purchased a different color bag for each child and color coordinated thier prizes and just hid them around the house or yard. That way it comes out even. With adult help you can put a paper in each bag with "clues" as to where to find them. If you want to go with a masquerad "ball" type theme, music and dancing is always fun! I have tons of ideas... it's hard to answer your question without knowing specific things. But here are some "rules of thumb" you may want to consider as I close this answer.
1. Make sure YOU can enjoy this with the kids too! It will be more memorable for your child, so make sure you are in costume too and can participate. If you have to and have the resources, call in some favors from other adults to help you. Delegate specific jobs so everyone knows what thier doing and do not have to ask you tons of questions.
2. Take many pics! Be sure to get one of your child with each guest, get an extra copy of those and send a thank you note with the picture in it. The parents will appreciate it and (probably not until thier older) the kids will too!
3. Send younger siblings to a sitter (family member,etc.) Focusing on this one and thier special day will mean a lot. Either send the older ones out to friends or let them stay to help, this teaches them about giving and making thier sibling feel special.

Birthday parties around Halloween are the best. Here are a few fun ideas:

Game: Gut Dip
1) For this Kids Party Game fill a large bowl or bucket with anything squishy such as pasta or mashed potatoes.
2) Hide small objects throughout the squishy substance.
4) They get to keep what they find.
3) Give the kids a set amount of time to put their hands in the squishy substance and find an object.

Food: Hotdog Worms
Hotdogs
Hotdog or hamburger buns
1) Cut hotdogs into thin slices and score the edges (a couple cuts per slice).
2) Boil the hotdogs. They will curl while they boil.
3) Serve the squiggly worms on buns (hamburger or hotdog buns will work).

And you have to have a costume contest if you are having the kids dress up!