Ok, real answer. I have a 7 and 5 year old.
Mario Maker. Great because you can download the levels (or alternatively make the levels) that suits their skill level. May not be as great if you are not involved in his life on a day-to-day basis.
Yoshi & Kirby: Easy games. Kids love.
Mario Kart: They love motion controls. Great pick.
Any Party Game: Overcooked, Mario Party, etc. All of theses games are premised on the same conceit - controls are imprecise, rounds are short and sweet and you have to play with others.
All Of the Discount Trash Games on the Nintendo Store: True story. My son is 5. There is a game called Battle 2048 or something to that effect. It is 2048. He fucking loves this game because you get to play pranks on the other players (hide their screen, flip their board, etc). He literally dies playing this game. He screams when he gets to prank other people. He does not understand math.