Germany has presumably decided to enact a specific law because tradition has made it acceptable for parents to mutilate their children. I'm sure Germany had perfectly adequate legislation under which prosecutions could have been brought, but even in secular nations iron age tradition makes some people think "these things are acceptable".
This law presumably reinforces that such assaults are illegal. It reminds people (police, prosecutors, public) that these are minors in care and ABH is ABH. I don't see how such acts can be defended on any basis.
Not having any way to check on German law and only reading about this in the press I suspect this is an amendment to their existing law on assault and not a new act.