This is quite a coincidence, because I was thinking about this the other day. I was thinking about having my sixth grade son do the experiment with Coke, Diet Coke, Mountain Dew, and Diet Mountain Dew. I thought that the different levels of caffeine might make a difference as well as sugar versus non-sugar sweetener. (Your daughter's one-factor experiment is probably better for a sixth grader than my two-factor idea.)

My hypothesis would be that more caffeine equals more chemical reaction, all else being equal. I have no idea how Pepsi and Coke compare on carbonation and sweetener, but since Diet Pepsi has less caffeine than Diet Coke, I'll go ahead and predict the Diet Coke to fizz higher.