My five year old has her own Mii too, I bought the Wii Fit Plus and Wii last summer and I was having so much fun creating my Mii that I did one for her (and DH too). The one frustrating thing is that since she's not great at reading yet, the instructions for some of the games are a bit beyond her and she doesn't really do what the game requires.
She has a great time with it though! She does the body tests too and I had to correct her height setting because she's grown so much since last summer that the Wii was starting to tell her she was fat. Five year old with body issues- add that to my list of stress I don't need in my life!