It is important to start flossing your child teeth as soon as they start to touch each other. Around two to two and half years 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 they may not have the skills yet to do a good job, you should supervise and give tips and pointers now and then 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.