Hamas said it fired at the bus because Israel had killed three militants last week, but it said Saturday that it did not know the bus was carrying children.So it seems that Hamas indeed targeted the bus. A previous article was mentioning that all the children but one got out of the bus at a stop before the attack took place, which would give some credence to Hamas' statement. But that's still attacking a school bus, I don't know how you can attack it without being 100% sure there are no children in there. Also, Hamas must have known that usually, you need a driver to drive a bus, and the driver of a school bus is hardly a Mossad operative or a definite member of the IDF.
You can read the NYT article, it also mentions an Israeli attack that killed civilians, while palestinians in the area witnessed no attack originating from there.
Everything's going well