I think the right time to introduce flossing between the teeth to your child is sometime after he/she has a close to complete set of teeth. Sometime between his/her first and second birthday, I would say, since your child would by then have at least 4 to 10 teeth. As soon as there are neighboring teeth that grow next to each other, I would say it might set a good habit to introduce the dental floss. Of course, younger children will require your assistance and there are a number of child-friendly (and very cute) dental floss sticks available in stores that might make the process a bit more attractive to your young child. Most children will not be too thrilled to have a string pushed between their teeth, or possibly even have their gums cut (first time flossers), but the truth is that the younger a child is when a habit is started, the more successfully that habit will be maintained throughout a lifetime. Most children will be able to floss on their own (more or less successfully) by ages 4 to 6 years (depending, of course, on maturity level of the child). My own son began using his own floss shortly after his 4th birthday and he had the colorful child floss sticks that I mentioned earlier.

Hi @iklilian, if your child's teeth touch each other, it is time to teach them flossing. This is very helpful in preventing cavities because it helps remove the plaque and food particles that are lodged between teeth.
Your child should be able to floss his own teeth when she is 9 years old. If you floss a young child, put her on your lap facing you.

This is how you should do it:
1. Cut about 18 inches of dental floss and wrap one end around each of your middle fingers.
2. Gently slide the floss between two teeth using your thumbs and index fingers as guide. Use a saw-like motion.
3. If you are at the gum line, pull both ends of the floss in the same direction to form a C shape against a tooth.
4. Pull the floss against the other tooth. Repeat this.
5. Be sure that you do this for all the teeth and also the teeth farthest back in the mouth.

They could get cavaties

You're correct about the oral hygiene part, however small kids will not be able to floss their teeth corretly untill they're about 8-9 years old.
When your kid is "old enough" you should probably floss your own teeth while your kid watches so that he or she learns how to do it properly.