Also, are Wii U and Nintendo 3DS suitable for little kids?

Yes.

Also get them Super Mario Galaxy. Its a Wii game, but the Wii-U is backwards compatible with Wii games and you need the WiiMote+Nunchuck to play.

A few of mine can that can be found here on GOG:

1) Terraria
2) Wacky Wheels
3) Superfrog
4) Driftmoon
5) Giana Sisters

Hidden Object may go down well if they're fairly new to gaming.

Get them Sacrifice .

Casual-ish: Check out "Botanicula" or "Journey of a Roach".
Non-casual: "The Feeble Files", "The inner World" or "The Night of the Rabbit". But since they let you safe at any time, they should work if you don't want your kids to spend to much time with it.

What age?

Maybe World of Goo?

for non-casual, i'd say broken age; for casual, i'd also go with world of goo.

Rayman origins might be great for kids. It has multiplayer and it's fun to play and not very difficult to learn to play.

Minecraft

Second this, any of the Rayman games would be great.

Nooo if you didn't add the Wii-U/3DS thing I'd say Rollercoaster Tycoon!

But since you said that, Animal crossing is usually pretty good, though I'm not sure how long it would hold their attention for.

What age do you mean?

Incredible Machine games.