According to this interview with someone from the Human Rights Watch, it is considered a war crime (if I understand it correctly):

The laws of war require armies to avoid deliberately targeting civilians, and also to avoid attacks that by their nature cannot distinguish between civilians and combatants. In particular, in Gaza, because it’s such a densely populated urban area, when you fire explosive weapons on a massive scale, it’s predictable that civilians will die. It’s predictable that children will die.