I have never been in an accident (knocks on wood). I am sorry you were but I am glad you and the baby are okay.
My sister was in a multiple rear-ending accident. She was about the 4th car from the rear and she was still responsible for the car she rear-ended. Because she didn't leave enough space between her and the car in front of her, she was responsible for damages to that car. The car that hit her was responsible for her rear damages.
Hopefully this mean guy will not have a choice in the matter. The cops on scene already charged the boy and that's that. They could fight it, but I seriously doubt they would get very far.

Good luck and I hope everything works out for you. I hope you are feeling better soon too.