What I would do if I were in that situation may be different than what I think I'd do in the situation if given, due to other, not necessarily selfish considerations, such as the concern for (if I had them) my own children if I went to prison. For that reason, I suggest one more assumption to idealize the moral problem: People don't depend on you.
With that above said, I can certainly say that at this point in time, given how things are now, I'd try to save the dying kid and then turn myself over.
Some people will shout pragmatical arguments justifying their decision to run away, but they are miserable people no matter what (again, we make our assumption that people don't depend on them). Also, If you turn yourself in because that would make you feel better, then you're not doing it for the right reason. You should turn yourself in because you just killed a child and almost (hopefully almost) killed another due to rashness on your side and your side alone.
Furthermore, note that I would consider the person who turns himself/herself in more free than the one who wouldn't, since their selfishness and desire for happiness is not enslaving them into making a different moral decision that they otherwise decided they would make as free thoughtful beings.
So yeah...I guess you can probably note I'm very Kantian.
