For the multiplayer games that will shut them all up at once, Wii Party U is better than Game&Wario. Though you should get both if they're cheap, and they should be by now.
Avoid Mario Party like the plague, this version is terrible.
Nintendoland has a great variety of games, and is often dismissed as being too tech-demo-ey. Stock up on Wiimote+ and nuchuck, they should also be cheap secondhand.
If your youngest is 6, try Super Luigi U instead of Mario. Whoever plays as Nabbit is invincible, so it'll keep the others from getting too frustrated.
Affordable Space adventures on the eshop requires real teamwork, and some of the roles require much less skills than others, so its got something for everyone.
And of course Mario Kart and Smash Bros. are no-brainers.
