Sorry but even if we assume somehow all other ways are exhausted and violence is your last resort, going after the kids and the festivalgoers is never justified under any circumstances. Maybe if Hamas wouldve directed the fight at the people that have bit to do with the situation like police or army who also use violence to enforce shitty policies.
But with the way things are and the actions Hamas did I don't have to think twice, I'll side with Israel and IDF. They are not perfect but still the way way better option.