That strength of emotion has little to do with Just World and everything to do with biology. Just World would be the way we rationalize it.
I don’t see why both can’t be at play here. Obviously saying we react so strongly to attacks on children is purely caused by Just World is a bit ridiculous, a strong drive to protect children is found in almost any mammal and is clearly beneficial to a specie’s survival. But Just World can enhance these feelings when it comes to children.
And yet in some situations, we’re denied the rationalization, thus when the tool for mitigating our emotional response is gone, the emotions are stronger. There are other factors, sure, but let’s not pretend the other factors are the only factors.