The word child can mean young person, but it can also mean offspring. The difference is small but distinct. A five-year-old boy is a child (young person) even if we have no idea who his parents are. And a middle-aged woman can say she has three children (offspring) even if they’re all in their 20s. (Hence the odd phrase adult children.)
We rarely think about this distinction, since we have a single word to cover both meanings. But it wasn’t always that way. I just learned that child has meant young person for pretty much its entire existence — back to Old English at a minimum — but the offspring meaning is relatively new, dating only to the 12th century. Before that, the word for offspring was bairn.
In summary: Isn’t that neat?