Healthy gums should not bleed and there can be a large variety of reasons why gums may bleed.
There are a variety of reasons why gums may bleed in young persons such as an accidental cut (from a sharp food like chips or aggressive flossing technique) to early gum disease (gingivitis). If you see bleeding gums, it is best to rule out the obvious causes first such as a cut, scrape, or other trauma.

If bleeding gums is associated with brushing and flossing, then sometimes this can be a sign of gingivitis. Gingivitis is gingival inflammation typically caused by poor plaque control, and typically resolves with improved and regular brushing and flossing technique. Twice daily brushing and once daily flossing is an important treatment technique for this condition - and if a child is under 8 years of age, then proper plaque control is typically achieved with an adult who has well-developed fine motor control.

Less commonly, gingivitis may be linked to other underlying conditions. Gums that spontaneously bleed should be assessed further, but proper oral care is important to rule out the most common reason for the condition (plaque).