Oh I would definitely second the angelfood cake with fresh berries!
But, I also think it's a good "lesson" to your kids that birthday celebrations don't HAVE to have cake. I'm not trying to be an ogre, there's nothing "wrong" with cake for a birthday, but a different perspective/experience might be interesting.
What about one of those fabulous
fruit arrangements? You could order one, or if you bought some of the shapers (melon ballers and things) maybe you could make one
together? Or even something really simple - put different fruits on skewers (melon, pineapple, berries), have yogurt/caramel to dip them in or something? Not sure how to work the candles in!
Oh, and happy birthday!