When my kids were a little younger, we played the Wii A LOT. The thing I really like about it is the communal nature of basically ALL the games; you are playing against each other, often in real time (rather than taking turns). I think it is still an excellent system for little kids and their non-gamer parents.
We got my (now 14 YO) son a XBox 360 w/Kinect last year, and that's about all he plays now. BUT it has been very difficult for us to find many games we can all enjoy (they are either set up for one person, or the controls are sufficiently complicated that learning how to get proficient takes more time than I'm willing to spend). The XBox "Live" stuff and the Kinect are truly amazing technical achievements.
For a 6 and 9 year old, I'd get a Wii, and be prepared to get something else in 3-4 years.
