I have nine nephews from 6 to 16. Seven of them are 9 or older. I finally decided to take the plunge, removed Windows and installed Hardy at my mom's place. When visiting grandma, my nephews saw it. That very evening I got calls from all seven requiring me -truly excited- to immediately install Ubuntu for them too on their own computers at home. They are absolutely fascinated by my claim that they won't get viruses and spyware anymore (they always get adults warning them severely about it and they're just tired) and they can't stop playing with Compiz settings either.
I wasn't sure they would like it at first so it was lovely to get those calls. Some will dual boot because they want their Windows games, but they're definitely going to do their homework, chat and surf in Ubuntu. You should see their faces as the try all those Compiz effects (even when that Advanced Settings manager is so poorly designed, without a single clue about what's going to happen when you change something). They're just inviting they're little friends from school to show them what their uncle just installed on their computers.
Their moms (my sisters) couldn't be happier. I'm happy.
We'll see how long the excitement lasts and how much trouble we get into when they require Windows-only apps, but so far all looks promising.