I see two ways history might judge this:
1. History could decide that the Geneva Conventions and current IHL with respect to combatant status, collateral casualties, and proportionality were simply wrong, and so everything done under current IHL is indefensible. Could happen.
2. It could turn out that the military impact of this strike was dwarfed by the direct civilian cost (in deaths and injuries to noncombatants and property they rely on), which we'll know more about in the coming weeks.
The fantasy is dangerous because it creates the expectation that military force can solve problems at minimal civilian cost. It can't. Wars are fought in cities, not on marked battlefields; unless we reduce ourselves back to a pre-industrial state, they will never be fought on marked battlefields again. Factor dead children into every war that ever happens from now on.
I don't like anything that is happening in the region. I don't think morally scoring Israel and Hezbollah is productive. All I have to say is that Hezbollah and Israel are military peers, and they are extremely at war right now.
(a) those which are not directed at a specific military objective;
(b) those which employ a method or means of combat which cannot be directed at a specific military objective; or
(c) those which employ a method or means of combat the effects of which cannot be limited as required by this Protocol;
So far as I can tell, this strike clears all those definitions. I think you may be reading 51(4) to be a prohibition on civilian casualties as collateral to military strikes, but that obviously can't be its meaning --- that would ban virtually all air strikes, for instance, and I'm pretty sure that isn't something the victors of WW2 were going for.
Am I misunderstanding the argument you're making? It's not unlikely that I could have!