Leonardo, that is the most unselfish birthday wish I've ever heard of. You deserve extra skritchies and catnip for that wonderful idea.
I did not know Sophie, but I know that she was greatly loved by ALL. I know that she will be having a wonderful party at the Rainbow Bridge and that her day will be made even more special because of your kind gesture.
Happy birthday Sophie and a SUPER stupendous happy birthday to YOU, Leonardo.