L.M.
Hello,
I know you have done a lot of work already, but I would get the same treat for the whole class. There are plenty of cake mixes that are gluten and dairy free - you just have to check the labels. I know it is your daughter's special day, but singling out one student may cause hurt feelings.
As a teacher, I see how children with food allergies feel. They often feel left out of fun celebrations. Also, double check with the school to make sure you are allowed to bring in cupcakes. Most states now have laws that do not allow sugar snacks into the school for any occasion.
I would save the cupcakes you already made for her birthday party at home, and get something else for the class.
Happy Birthday to your daughter,
L.