The 300 bones of a child are actually one bone, which is split into two ends via underdeveloped cartilage as well as infused sutures. But near puberty most sutures fused, and the long bones have had the cartilage become actually bone. So on average the safest answer is 206.
However because in medicine and just overall human anatomy nothing is a for some sutures or bones may not have fused, the child may still have several more than what they are supposed to.
