Sorry to hear of your misfortune...
I would be cautious about saying that you were backing up at the same time, as I have heard insurance companies automatically divide the fault because so many accidents occur when backing up and there are inherent blind-spots, etc. Don't lie of course, but as the other gentleman said, don't implicate yourself unnecessarily if you can avoid it otherwise. Also, the insurance companies will likely require a police report to legitimize the accident.
Good luck, Adam Bush