I assume you'll be making a scratch cake. This is from the site below to give you an idea of the quantity of batter in a 2 layer cake mix (I thought it was more, actually.)

20~ Packaged, two-layer, cake mixes yield between 4-6 cups of batter- so always measure. Usually, one 2-layer cake mix will make any one of the following: 2- 8" round layers, 1- 10" round layer, 1- 7x11x2" sheet cake, 1 character cake, or 1 Wonder Mold cake.

I like the idea of the chocolate chips for the center & that's something your GD could definitely help placing - if she or Grandma don't eat all the chocolate chips.

I made Pixistix's Hot Milk Cake & it's a winner. It rises high so might work for that cake pan.

Pixistix Hot Milk Cake
Old fashioned, simple, melts in your mouth!
4 eggs
2 cups sugar
2 1/4 cups flour
2 1/4 tsp. baking powder
1 tsp. vanilla
1 1/4 cups milk
1/2 cup (1 stick) butter

Beat eggs and sugar well. Combine flour with bk. powder and add to egg mixture along with vanilla. Heat milk and butter together until butter melts; gradually add to batter, beating until well blended.

Also check out Ilene's mom's chocolate cake. It's posted on this forum.

Pour batter into a greased 9x13 pan. Bake at 350 for 30 to 35 minutes.

Here is a link that might be useful: Baking tips