That is very tragic. Reports on what happened are early, but it seems Israel bombed a Hamas encampment in a safe zone, which is allowed under the rules of war when militants try to shelter among civilians.

Then a fire spread to nearby areas due to some tents being flammable.

Tragic, but part of a war. When a military unit is found, they can be attacked as long as expected casualties are "proportionate."

As with earlier Israeli strikes on tunnels that were so extensive that collapsing the tunnels also collapsed residential buildings on the surface, sometimes there are more casualties than you can predict.