It is unusual to humans and it is often speculated that the purpose is to be sexually attractive to males.
That's a theory. A theory that is of zero importance to children. Children don't need to know what scientists speculate about.
Why do men have beards? Are beards to make boys look grown up and attractive? Perhaps that is why they evolved, but fact is, we don't know for sure.
"In puberty, some girls grow bigger breasts than strictly necessary, and boys grow beards and get lower voices, that's because hormones cause it. We have no idea what that's supposed to be good for, perhaps it is just a thing that happens" would be the honest answer.
Or, you know, just say "because hormones" and not go into detail.