Funny you should mention the Birthday cake remix. That is the one I got when we were in there for my birthday, I thought the big cup looked about the right size and then they loaded that sucker up. I got about 1/2 way through and had to toss it.
We now either buy a small one each or get a large and split it.
I am really into making things for myself and after watching those shows this week and seeing your recipe I am going to have to do it. My oldest will get a kick out of it as well. In the brewery I always had homemade rootbeer and gingerale on tap and he woudl come in on Saturday's and help me make it.
Here is a review from one of the beer critics who came into the brewery while I was still running it.
http://www.realbeer.com/nmvbp/highnoon.htm