I am so sorry for what your family is suffering right now! I do feel that your children will be OK because they bounce back pretty well at that age. I worry about
you feeling guilty and your son feeling guilty....do you see the connection? Imagine (Mom feels responsible because she didn't insist on seat belts.....son feels responsible because he had the accident...)
Perhaps some family counseling about guilt could be of benefit. Please don't get me wrong, my WHOLE family suffers from this overwhelming guilt complex. I see signs of it in my daughter too. She was worried because I left the house the other night without taking her (her father was home) and she thought she had done something wrong to have to stay home....guilty...I had only gone to help with a computer problem at my mother's and didn't want to have to drag her out that late. It was not punishment and had nothing to do with her, but she felt guilty and couldn't even explain why.
My psych has helped me with this a lot. He has given me the ability to say no without being consumed by the guilt that I could have prevented or should have done something when I never had control of the situation anyway. Your son is feeling guilty when he didn't really have any control....the deer ran out in front of him...period. It was not his fault! You had no control over this accident either and did the best you could with your children. You obviously love them very much.
Please quit kicking yourselves for something that you had NO control over...and consider letting your son read some of these posts. He should never allow another human being to "make" him feel guilt. That relationship is doomed to fail if she continues to manipulate and he allows it....and he will be left with the scars to prove it.
A very wise person once said that nobody can make you feel bad about yourself unless you let them.
I wish you all the best!