Ah, now I got it: Newsfox. BUT I now see there hasn’t been any development for one year… so I’m not sure it’s still the best choice!
I also remember testing another one named Sage, but it behaved somehow differently than Vienna.
Newsfox (for firefox): http://newsfox.mozdev.org/
I don’t have Windows, but Xara Xtreme and RealDraw have good reputations. That said, they’re complex, fully-featured programs and may not be suitable for kids. I’ve used SerifDraw and it’s very capable, but the interface was typically Windows—rows and rows of icons and a confusing set of floating palettes.