Lucky you!
I just did the same thing last christmas for my nieces/nephews...
luckily for me, their dad is a linux guru, and he preconfigured all 7 PC's with ubuntu, beryl, etc.
We had them all set up in each of the kid's rooms, and made them solve a puzzle... (ages from 15 to 10)
It took a bit for then to finally realize they each got their very own computer, but once they did, it was awesome!
I'm so lucky I don't have to be tech support, I'm just the nice uncle who did it...