It's been said already. I think nobody particularly agrees with it. Kids aren't born knowing how to stay out of trouble, and parents aren't all-seeing gods. Once in a while something bad can happen and it won't be anyone's fault. Yeah, there are some parents who are all-around oblivious to what their kids are doing, but even the sensible ones have to blink sometimes, and that's all it takes for an accident to happen. I don't think it's fair to assume that this was a case of parental ignorance or neglect.

The rider who stopped to avoid hitting the kid is a hero, and the guy who was tailgating him was the only one responsible for any injury and damage that actually occurred.

Someone asked how you can not know that you're being tailgated. Honestly, this happens to me all the time. I'm slower than most of the bigger-wheeled bikes in the bike lane, and a well-tuned bike coming up behind me does not make enough noise for me to hear it. I will frequently discover someone in my peripheral vision before I ever heard him, because he didn't use bell or voice to indicate that he wanted to pass.

(In contrast, my seat suspension squeaks and my chainguard is bent and rubs against the right pedal, so if I ever was able to catch up with someone, they'd hear me coming.)