Having been in a couple of those type of situations myself over the years, let me give you a piece of advise. I know free advise is worth what you paid for it, but I offer it anyway.
First life is full of risks and things we do not understand. Do not play the what if game. You did the very best you could, and from the sound of it so did everyone else. They are ordinary people that did all they could. Even if there were something that could have been done differently with the vision of hindsight, you and everyone there had split seconds to react.
Be thankful this wasn't worse. It could have just as easily been two lost as two saved. If you could have asked the mother, if one of the two of them had to go right now you or your child, who do you think she would choose? Fact is you don't have to ask. She proved it with her actions and the child will grow up knowing she loved him more than life. Remember the love not the loss.
When kids suffer losses young in life, they will face special challenges. Yet with those special challenges come special opportunities, and those opportunities create special gifts for everyone around them.
You will not forget what you saw today. Don't even bother to try, and I am not sure it would be a good idea if you could. What you will do is find a way to live with the experience. Even the saddest days can turn into something positive if those that lived them look for a way to make it so.