You're being a good dad to remind the kids about their mother's birthday. You are teaching them to honor her and that is what you should do. However, the cake and song is over and above. Unless you see it as that you are still her husband....then, I see it. But, when you are divorced you no longer create parties for the person who divorced you. You will be focused on your own life that she abandoned....again, unless you are hoping to reconcile after the divorce. All things are possible.
