Leigh,

An ice cream cake doesn't have to be complicated. Bake her favorite cake -from a mix or whatever - in a 13x9 pan. Freeze it. Soften her favorite ice cream just until you can scoop and spread it. Working quickly, cover the frozen cake with it - a half-gallon probably will be enough. Don't cover the sides, just the top. Freeze it until the ice cream is firm again. Again working quickly, frost the whole thing with (defrosted) Cool Whip, and freeze again. When the Cool Whip is firm, you can decorate it, wrap it, and freeze it until the day of the party. Let it sit out for a while before serving so that the cake will soften enough to cut. My kids love this. Just don't serve it on flimsy paper plates!