I assume it's milk and / or eggs you're hoping to avoid? There's a brand (Cherry something??) that makes different allergin free birthday cakes. You can find them at Kroger (with either the health food or dietetic food), Target (in the cake mix aisle) and health food stores. I made one for my son's birthday since one kiddo was allergic to eggs. It was yellow cake and was really good!! A good friend (and cake snob) asked me afterwards for the recipe since she had been craving it for weeks. She couldn't beleive it was a mix. She prefered it over my home made devil's food cake!! It was very moist. Their frosting mixes use butter but also give dairy-free instructions. The crisco recipe below is also probably a safe bet.
Not sure how you're decorating the cake, but there are some great web sites out there for suggestions/instructions. I probably still have links so if you wnat them, let me know.
Good luck!! Don't worry about the cake. A home made cake, no matter how imperfect looking, always beats a store bought one in my book.