I have an idea in my head for my DD's party with a 2 layer square cake (no real idea why). I was wondering how many servings could I expect to get from it for 8-9 year old girls?

She could have anywhere between 10-20 people (mostly kids). It is a Doll fashion tea party with American Girl dolls. I was thinking of doing something two layer to put stars on the sides, but am wondering if a basic 1 layer 9x13 would be better for the size of the cake (doing a double layer would probably be too much cake).

Ideas?

You will actually get more servings from the 2 layer (4" tall) 8" sq.
Depending on what size you cut the pieces you can get:

1x2x4 = 32
1.5x2x4 = 24+ 4 servings 1x2x4

9x13x2 pan yields 24 pieces 2x2x2. That's the same amount of cake as the 1x2x4 pieces or one (average size) cupcake.

an 8x8 cake could serve 16 if you cut 2x2 inch pieces, but if it is a taller cake, you can cut smaller pieces, 1x2 inches, and have 32 servings because they get the cake in height- so it all depends on how big your slices are.