Got a Wii here, love it.
Definitely get at minimum 2 controllers with nunchuks, although 4 would be a better idea. So much fun doing party games with everyone when they're over. Also suggest getting a rechargeable docking station, batteries get expensive!
Some games your kids will like: Super Monkey Ball, WiiPlay/WiiSports/WiiFit, Lego games, Mario Kart, Mario Galaxy, Super Paper Mario (might be a bit advanced at 6 though).
Lastly: Wii homebrew! Check it out. You can watch dvd's, play NES/SNES/any emulated console game, play your 'legal backups,' you name it. Super, super easy nowadays.
Have fun!