They’re welcome to retaliate. They’re just not allowed to indiscriminately bomb civillian infrastructure.

Here’s a field manual that details the rules and has some advice. There are a whole host of rules protecting civillians hospitals, but in the case where Hamas is using them as military bases, I’d say they can be considered primarily as human shields, though I’m no expert, and even if that’s not the case they’re still civillians and therefore protected. According to paragraph 2-20, “feasible precautions” must be taken to reduce civillian harm. This means bombing is pretty much out of the question, but there are still plenty of other ways to get at Hamas, such as SpecOps, sieges, and diplomacy. It’s a difficult situation, but that doesn’t mean you get to kill civillians with impunity.