I have a household problem solving book here with me that actually has a recipe for toothpaste:

1/4 cup of arrowroot
1/4 cup of powdered orrisroot
1/4 cup of water
1/2 teaspoon of oil of cinnamon
1/2 teaspoon of oil of cloves
1 teaspoon of ground sage

Combine all of the ingredients. Add more water if the paste is too thick. Store in an airtight container at room temperature. Makes 1/2 cup.

Sorry, I am not sure how much those herbs and spices cost.

Source: Reader's Digest problem solver

One that seems more cost efficient and better at cleaning would be this one:

--quote--

1. Baking soda (works as a natural cleansing agent and it polishes and whitens your teeth as well!)
2. Table salt
3. Hydrogen Peroxide (naturally disinfects your mouth)
4. Stevia (optional as sweetener)
5. Peppermint oil (optional)

--end quote--

http://www.christianpf.com/how-to-make-natural-toothpaste/

The directions for making it, is at that link. I think with the flavor and sweetener, the kids wouldn't have a problem with it. I have actually used baking soda alone and peroxide to clean my teeth in a pinch.

I've also heard that in the past, people brushed their teeth with chalk. This article talks about that, and it does include charcoal and ashes as well but doesn't tell the process:

http://kidshealth.org/kid/stay_healthy/body/teeth.html

I also remember an episode of wife swap were the family used something else to brush their teeth.. it was homemade butter and clay. I do not know if this is good or not. I can not find a link with instructions and if it's actually good for your teeth or not.

Baking Soda and water. Cheap, easy, doesn't taste the best, but it works well.

There is one thing i remember from the history books in the UK that people used during the wartime era when rationing was the way to do your shopping and certain items were beyond reach due to them being too expensive to make. From what i remember the people used to use SOOT, soot was the black stuff that was found just up inside the chimney breast of the fireplace and if memory serves me correctly they mixed it with a little drop of water and used the paste that it made to brush their teeth also they used their fingers as toothbrushes as even then toothbrushes were scarce, well i hope this helps you make your own toothpaste.

Make sure your water is fluoridated if your using all natural, unless you add fluoride to your paste. Its very important to have fluoride in your toothpaste. Its controversial, but all claims saying its bad is bogus. Its only bad when teeth are forming, because it causes fluorosis, hence, thats why you dont give fluoridated toothpaste to children under six. There is a toxic level of too much fluoride, but this will not occure with a normal daily intake in your water and toothpaste.