So you are suggesting that the IDF knew it was children and thought "why not" and fired on them?
Ok, so let's talk about this in a less "IDF IS ALL ABOUT KILLING THE CHILDREN" vs. "IDF IS WONDERFUL DEFENDERS OF FREEDOM" kind of way. It still looks bad for Israel. So they believed there was a weapon placement there. Let's take that as a given, even though presumably if there were, the press who were filming this happening would have noticed it. Let's also take as a given that Hamas is using civilians as shields. Basically, full credit to the Israeli side of the dispute.
So they shelled it, presumably hit dead on what they thought was a launcher of some sort. They saw people scrambling away. They turned their guns on the people fleeing, who they know full well might be civilians trying to get away from the shelling and not operators of the launcher. But still, they turn the guns and fire at them.
This seems completely unnecessary, even if they *weren't* kids. The civilians in Gaza are Israel's responsibility, whether they like it or not, because they are an occupying force. That they've removed themselves from securing the lives of those they occupy does not remove this responsibility.