Give them American citizenship, for this reason if for no other; when Belgium and Germany and France are all consumed by the Islamic State, when it imposes sharia law, they'll have someplace to flee to. Yes, it's possible that Belgian citizenship might get them entry to the US when darkness falls (and it may still happen here anyway) but the USA hasn't got the best record for accepting political refugees from Europe.
If they are American citizens, they're part of the family. And as Erma Bombeck once wrote, being family means that when you knock on the door, they have to let you in. (And if that wasn't Bombeck, it should have been.)
Perhaps it's unlikely - almost as unlikely as Islamic terrorists shooting up a French magazine office and a kosher deli. But it's worth considering.