Oh mugs, I can't even begin to imagine the shock and horror you must be feeling! The only suggestion that I can think to offer you is to compartmentalize. If you can, take all of your thoughts and concerns about your relationship and your worthiness to exist and put them aside to think about later. They aren't going to go anywhere (unfortunately). And the girl's family will get help and support, don't worry about that. You may find yourself being involved with the family, since you witnessed the accident, but they will be getting lots of other support as well.
Focus your energy on getting yourself and the kids through this tragic experience.
Sending you strength and compassion...