What would be good starter supplies for a tween that wants to start cake decorating? I'm a hobby baker, so I'd love some professional opinions. I would definitely say a star tip, writing tip. What else?

Wilton has some starter kits. Some come with templates for learning how to make flowers, do lettering, scrolls, etc.

Here's just one example:
http://www.amazon.com/dp/B00004S7YF/?tag=cakecentral-20

Hope this helps! Good luck!

I think a gift basket would be nice filled with various items such as decorating bags, a set of decorating tips(star, drop flower, round, rose, leaf, & basket weave), beginner's decorating book, apron, cake pan, wooden spoon, and spatula. A cute idea would be to use a large mixing bowl as the basket and a recipe index card as the gift tag.