Flossing should be started when your child is around 2 to 3 years of age, under the direction of your child's dentist. Prior to this age, flossing is not necessary. Children usually need assistance with flossing until they are 8 to 10 years of age.
Brushing teeth properly and consistently helps to remove most dental plaque; however, brushing alone cannot remove plaque that is located in places that a toothbrush cannot reach, particularly in-between teeth. In addition to removing plaque, flossing also helps to:

Flossing should take place at least once a day for two to three minutes each time to be most effective.
Regular, consistent flossing is the single most important weapon against plaque, perhaps more important than the toothbrush. The different types of dental floss include the following:
Your child's dentist or other oral healthcare provider can provide a demonstration of any of the following flossing techniques. They include:

Flossing tools, such as a prethreaded flosser or floss holder may be helpful for people who are just learning how to floss, individuals with limited dexterity in their arms and/or hands, or persons who are flossing the teeth of someone else (particularly a child or disabled person).
Irrigating devices, such as waterpicks, are not considered a substitute for brushing and flossing. These devices may be effective around orthodontic braces that retain food or in areas a toothbrush cannot reach. However, they do not remove plaque.