You can't magically excuse yourself from ignoring or forgetting to watch a child because you have multiple children. That's YOUR burden to watch all of them.
If what I heard was true and the kid was actually talking like he thought people were gunna jump in either way (heard off of a witness's thing), that mother has even less of a reason to justify that 'oh mistakes happen'
Also if it's true that there were multiple fences, and they're large fences that are spaced apart, you'd have to be EXTREMELY distracted not to catch him running out. But I don't know how the fences are laid out, I've only heard, so.