How do I make icing or marzipan without using anything with cornstarch in it? All the icing sugar I have seen has cornstarch in it. She says she uses goat's milk in place of cow's for baking, will that be ok? Making a chocolate cake that is going to be carved...

Also what would you charge for a small (but carved) lightening mcqueen smash cake with those dietary restrictions? Am I supposed to charge more for cakes like that? I am a newbie!! Please help me!
Thanks!

Personally, especially depending on the severity of the allergy...I wouldn't even try. The equipment you use and your kitchen can still have traces of those ingredients on there. I don't do any cakes for allergies or anything because under normal circumstances, all that stuff is still in my kitchen.

3GCakes makes a good point -- you should definitely discuss that with your customer.

A meringue buttercream might be an option if you can find a good butter substitute. If you must use a shortening-based frosting, you could try making super-fine sugar by running granulated sugar through your food processor (or blender). It won't be as fine as powdered sugar, but it will be closer.

Good luck!