I'm pretty sure edubuntu is probably your best option. Loads of fun 'tux' related games and some parental control applications.
You can also 'try before you download' online via a Java applet on their website too.
Your best bet may be to go with edbuntu because:
Based on Ubuntu, known for ease of use and stability. Also the huge selection of packages available.
Looks like there are educational packages based on the child'support level that can be downloaded. http://edubuntu.org/download
It will get them used to the desktop dynamic which is still prevalent in education and business. Though cinnamon or mate reflects this better than unity.
I would probably get edubuntu. Remove unity. Add cinnamon. Tweak where necessary for your child'support needs.
Then purchase and android tablet to supplement.
One a side note, we signed my 3 year old up for ABC mouse (a Web app and service) it has worked wonders. It has an android app as well.
It'd probably run WoW but I don't know firsthand. I suggested linux cause it's free and gives you a few options that are going to be faster and less bloaty than vista. And having access to something like edubuntu is good for kids. Anyway if you ever want to give it a try and need tech support, hit me up. It's really as simple as burning a CD and popping it in the drive. Good luck with the new system.