Flossing is only necessary once your child has two teeth that touch. This usually first occurs with the last two molars (around age 2). To tell whether your child needs to floss, try this simple test: If you can't see a space or see the gums between a pair of teeth, slide a piece of floss between them. If it sticks a little you know the teeth are touching and it's time to floss.
At first you'll need to help your child, since she won't have the dexterity to floss on her own. As a general rule, when your child can start to learn cursive (around second or third grade), she's skilled enough to floss solo. Some children are ready to go it alone sooner, but you should supervise at first to be sure they can reach their back teeth and are doing a thorough enough job to take over this important responsibility. Flossing gets rid of bacteria and plaque buildup that can lead to inflammation and gum disease over time.