12x18 (called a half sheet by many here) single layer will serve 54 (2x2x2) pieces.
A two layer 12x18 half sheet will serve 108 (1x2x4) pieces.
To serve girls and dads, I'd probably make a 2 layer 12x18 cake.
If cake is the only dessert than, yes I think you need more even if its double layer. If its a banquet and there will be cookies, brownies etc. I think you will be ok with the 2 layer that size. You can look up serving charts for any size pan here on cake central or just google it
Standard serving size for a 2" high cake: 2"x2"x2"
Volume of a standard serving: 8
Volume of a 12"x18"x2" cake: 432
432/8 = 54 servings
Standard serving of a 4" high cake: 1"x2"x4"
Volume of a standard serving: 8
Volume of a 12"x18"x4" cake: 864
864/8 = 108 servings
Here is a thread with some good info on how to determine servings on sheet cakes ("Do the math"), plus a link on the history of sheet cake sizes and why the 12x18 is considered a half sheet: http://cakecentral.com/cake-decorating-ftopict-670996-math.html
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