Do I really have to do sth. like this?
That is one strategy, yes.
On bi-directional relationships there is an “owning” and a “non-owning” side of the relationship. Because the owning side in your case is on Child, you need to set the relationship there for it to be persisted. The owning side is usually determined by where you specify @JoinColumn, but it doesn’t look like you’re using that annotation, so it’s likely being inferred from the fact that you used mappedBy in the Parent annotation.
You can read a lot more about this here.