There's absolutely no harm in it, unless you're, say, another kid at Moddey who's lost their parents. Like Arista and Hector. Why don't the Longbottoms just adopt everyone while they're at it? I honestly could give two knuts for what the Longbottoms do in their personal life, but I imagine that some of the other parentless kids who look up to them as parental figures might feel quite differently.