No worries.
HSM is just karaoke. There's a great HSM karaoke CD if you already have the machine.
Hannah Montana is a dance game. No one should buy a dance game that doesn't use a dance pad IMO. It's just stupid to wave your arms around without having to move your feet.
Review:
http://wii.ign.com/articles/837/837767p1.html
I think the Hannah Montana game would get super old super quick for a 10 yo. I know that she's the perfect age for the REAL Hannah, but at 10, she should be playing more substantial games like Mario Galaxy or Super Paper Mario.
BUT I'm sure she'll probably like it for at least a little while. Just not enough for me to spend $50!
I honestly think the HSM game is more worth the cost, and is different than just a karaoke machine.
Here's an idea...rent it! Give her a chance to play it over Christmas break or something. Then, if she still wants it, but it for her. Better yet, rent both games for her!
I hope this helps somewhat. It's hard to resist buying those games, but they just aren't worth it.