If you want to bake something, I suggest getting a miniature muffin tin and using Pamela's gluten-free vanilla cake mix and Pamela's GF chocolate cake mix to bake some cupcakes everyone can frost. You can also buy miniature bundt cake pans, which make adorable little bundt cakes. We did this at my daughter's 10th birthday party, and it was a big hit. (See pictures.)
We bought Betty Crocker's GF white frosting (check labels), but be aware that this stuff isn't usually dairy-free too. So if you have a dairy-free person in your household, I recommend using powdered sugar (confectioner's sugar), Earth Balance spread, and soy milk to make your own frosting. Also check to make sure your food colorings are gluten- and dairy-free.
Candy sprinkles are usually safe, but it's always a good idea to check those labels too.
We also included marshmallows, GF hot chocolate (but that's not dairy-free, usually), strawberry guava juice, lemonade, and K-Toos GF and DF cookies on our tea party menu.
I had the girls frost the cupcakes and bundt cakes, so they had a lot of fun with that (although expect a mess). By frosting the cupcakes themselves, each person at the tea party takes pride in what they've made. You'll have fewer problems with non-gluten-free eaters turning their noses up at a cupcake they frosted themselves.