Israel told civilians to evacuate to that area, stating it was a 'humanitarian' or 'safe' zone. They therefore knew it had a high density of civilians, including children and other vulnerable people.
Israel also has made a big show of its efforts to evacuate people, using this as a key part of its defence against the case in the ICJ. These notifications have continued, telling people to leave areas in advance of operations there. (Although there's an awareness that these notifications are often misleading).
Israel had the knowledge of a high density of civilians, the capability to communicate evacuation instructions, and likely the time to tell people to move out of the area (it is extremely unlikely that any opposing force in the area would have posed an imminent risk of severe harm to Israeli civilians). They chose not to. They chose to fire missiles at an area they had not evacuated, and in which in fact they had created a high civilian population density.
They are responsible for that decision.
When you fire a missile somewhere, you are responsible for what that missile does. Things like "if this hits something flammable..." need to be factored into that decision to fire the missile. And tents, cooking equipment, and civilian populations in general are quite flammable.