I know--I know Thor is a baker by trade and he will have something for ya. Sorry I don't bake at home much at all and the recipes and formulas I use call for 50 pounds of this and 30 pounds of that.
When the boys were little we were in Cub Scouts and we used to have a Father/Son cake competition and auction. That was fun! We made cakes to look like volcanoes (tallest cake award winner), one that looked like a plate of spaghetti and meatballs and other fun stuff. It was stressful with everyone knowing what I do for a living though.
Have a blast with making the cake, your niece will always remember it even if it doesn't come out the way you planned it. We are always our own biggest critics


Thor