One other option too is any of the sleds built on the Skidoo Freestlye chassis built 2006-2009. I have a 2007 Tundra 300F (300cc single) that my 9 and 10 year old can drive. Electric start, reverse, heated handlebars, a huge windshield and a 16 x 136 track. I don't think it weighs 400lbs.
Best part of this sled is its geared like a tank so it is vey easy to learn the throttle. You can putt around all day at 10 miles and hour and its happy to be there, but I wouldn't be suprised if you can get it to 45-50mph too.
Its skis are about 36" wide so its a bit more tippy, which will actually be fun for those powder turns when they get a little older!
OTM