Most children are unable to manipulate a toothbrush properly until they are seven years old or more. Encourage them to try by all means, but make absolutely sure they clean theirproperly. Clean their yourself, then let them try afterwards.
Make sure children have a brush with a small head, which will easily reach to the back of the mouth. Show them how to remove food particles from between the teeth, with an up and down movement, rather than a simple polishing action.
Impress on your children the need for at least one thorough cleaning each day; this is far better than several cursory attempts at cleaning throughout the day.
Try to educate your children to see the dentist as a friend, who can help them avoid the unsightly teeth they see in many adults. It is not too early to take the child to the dentist when it has only a few teeth; even as early as six months.