Not silly at all.
I think it is wonderful that you and your grandmother
are going to have a party for the pups.
Like others's said check on the internet for
doggie recipes, and I bet if you check the
Yellow pages you will find places that sell
bakery goods for dogs too.

I think the hamburger meatloaf cake is a great idea.
Instead of the mash potatoe frosting you
could decorate the hamburger cake with cut up hotdogs and make a design.
I think your party will be a lot of fun.