Support is key: I don't support my (extremely non-technical) mom's Windows laptop or her iPhone because there's nothing for me to do. (My mom had a stroke and is mid 70's.) She wouldn't be able to use anything other than Windows because the retraining burden would be too great. I would expect most 8-year-olds to easily move between any OS because they're far more adaptable than ye olde fuddy-duddy adults.

PS: Which Linux distro?