Children need to have their teeth flossed daily as soon as thier teeth start to touch each other. Normally this is around two to two and half hears old Flossing makes a big difference in the prevention of cavities.
Normally by the time your child start kinder garden he or she should be able to start flossing their own teeth, though you may need supervise. while your child is learning how to floss, try and give tips and pointers to encourage good oral health care.
Just like adults a child should floss their teeth every day. A good time is right before they go to bed but as long as they are flossing each day that is what is important.