Usually people experience hair loss when they get older or due to hormonal issues but this is not always the case. Even children can experience balding or hair loss, but for a different reason. Usually this is caused by alopecia which is a medical condition which prevents hair growth.
Most of the time this is a treatable condition especially if it is caught and treated early. However, if it occurs in patches or only in a specific spot, then it may be caused by ringworm. Regardless of the reason, it is best that you take your child to the doctor so they can find what is causing it.