This is another tricky concept. How to teach your child correct Circle behavior. Heck, I'm still working on this one. I have to remember that children are especially sacred to the Goddess, and that She is not going to be too upset if my little one wanders in and out of the Circle from time to time.
Young children have very short attention spans, so a long and complicated ritual will quickly get boring to them. So when including your child in ritual, make sure that it is short and simple, with a dash of fun. That is the best way to keep your child interested long enough for you to begin teaching them simple Circle ettiquette.
Try dancing and holding hands in a circle with your child. Play "Ring around the Rosies" with them. That little game is in actuality a very old game dating back to the time of the Plague. Kids like to play games and sing songs. This is a great way to get your child interested in ritual and circle time.