Kid Friendly UI / Shell

Hello, this Christmas I am building my nephew a small lightweight PC for his homeschooling. He is 6 years old and great at his computer classes, however they don't operate under a normal windows environment and I want a solution that would help me control the UI/Shell allowing only a few basic things to be accessed.

Namely specific software I install for his use (his Disney games and whatnot) as well as a few select websites (NickJR.com, PlayhouseDinsey.com etc.) while also allowing me the ability to slowly introduce him into basic computer usage (what the start menu does, etc. VERY beginner Windows education)
I've tested a number of products from Stardock, including WindowsBlinds, DesktopX, IconPackager, CursorFX. A few have some promising capabilities (namely DesktopX) however I don't want to slap down $60+ bucks on a software suite that won't allow me, or at least cannot work in conjunction with; some type of software that will not only limit his ability to access potentially harmful aspects of the OS (messing around with the hardware manager or something) even right down to even customizing or disabling the right click menu on the desktop.

Surfing Deviantart, Wincustomize.com and a few other sites I found, don't really turn up any premade UI's that will really work for my needs with any of the aforementioned software. Basically I'm looking for a resource, a website or a forum community that might have solutions that would work for me in this capacity. I own copies of Vista and XP x64 & x32, as I have not installed the OS on his new system yet I can install whichever I need to based on what software turns out to fit my needs.

Basically I need a kid friendly windows theme, UI, or shell replacement that can allow me specific control over what the user can access, while still being customizable in an aesthetic sense, I would like it make it fun and appealing for him, something that maybe I can even change the theme on from time to time to mix it up (e.g. Maybe a Spider-Man theme one week, and Disney/Pixar Cars theme the next)

If anyone has any idea where I can do some digging or testing with software I haven't found yet, please post it here, and any other comments or suggestions you may have, I appreciate your time, thanks, and a Happy Holiday!