Might wanna hurry. Most stores don't even have stock of the system anymore and the game racks are getting more and more bare as we get closer to the Switch.

For the older kids, no one has mentioned Fatal Frame 5. Awesome game with tons of replayability (multiple endings, costume unlocks, ect).

(More for the Younger kids)
Nintendoland
Mario Kart 8
Splatoon
Yoshi's Wooly World
Kirby Rainbow Curse
Wii Party U
Mario Party 10
NES Remix 1 and 2
Dr. Luigi
Zelda Wind Waker HD
Pokken Tournament

(More for the older kids)
Smash Brothers Wii U
Donkey Kong Country: Tropical Freeze
Xenoblade Chronicles X
Super Mario Maker
Pikmin 3
Zelda TP HD
Assassin's Creed IV

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.