I made my son an applesauce bundt cake (from scratch with honey as a sweetener), with no frosting for his first birthday. I served that to everyone as I believe that your message will only be stronger if you're consistent with the view that eating lots of sugary stuff isn't good for anyone.
Others have suggested Whole Foods, but all those mixes or cakes will have as much sugar as Betty Crocker, although they don't have all the other additives, so they're slightly less toxic. I like the idea of a fruit based cake that someone suggested. There are also those fruit tortes/pizzas, where you have a thin crust, then a layer of cream cheese (or your favorite non-dairy alternative) topped with fruit. You could also get an angel food cake and top that with fruit.
Here's a good link on mercola.com that discusses a sugar free event for kids (you'll have lots better behavior overall):
http://www.mercola.com/2004/dec/4/sugar_free_birthday.htm
I will say that I have eased up on the sugar restriction over the years, as unfortunately, it is very difficult in our society. I just work on limiting the amounts. And, making sure that we always have lots of fresh fruit on hand as an alternative. My younger son, who has the big sweet tooth, is perfectly satisfied with a cup of mixed fruit in the evening as he is with his favorite chocolate chip cookies.
And, my kids are generally extremely good about not eating too much sugar (i.e., by allowing them a little, they don't feel deprived). They still have candy left from last halloween and Easter, which we just throw out before they go trick or treating this year.