Oh man, where do I start?
The usual big ones in Super Mario Odyssey, Mario Kart 8 Deluxe, Super Smash Bros. Ultimate, and Splatoon 2 are the good bets; solid, polished, and fun while still being challenging (for most of them). Splatoon 2's DLC, the Octo-Expansion is very good and more challenging than the main game's single player story mode. For other exclusives, Bayonetta II is a great one; it's a pretty short game, but it is a huge thriller and far more forgiving and refined than the first game, and its replay value is down to trying to find all the fights and items and getting better ranks in the fights.
In terms of RPGs and strategy type games, that are also exclusives, Mario and Rabbids: Kingdom Battle is a big surprise to me and I actually enjoyed that quite a lot more than I expected; it is nicely presented and well detailed and I like the strategy aspect to it. Even though I don't like the Rabbids, they're nowhere near as annoying as I expected; the collaboration somehow works well! Octopath Traveler is a fantastic game to go for; beautiful music, great presentation and very much a JRPG through and through (if a tad grindy at times). It is long so you'll get your money's worth no doubt! Pokemon Lets Go are pretty good and are nice to introduce your kid to the world of Pokemon, and maybe get prepped and ready for the new games Pokemon Sword and Pokemon Shield later this year.
Outside of RPGs and stuff to introduce to your kid, Captain Toad: Treasure Tracker and Kirby Star Allies would be good starters; they're easy to complete, but Captain Toad is trickier to 100%. Same for the upcoming Yoshi's Crafted World, in that it's likely to follow the usual blueprint, for better or for worse, of being easy to complete normally, but hard to get all the items.
There's so much more that I'm sure others can help you on though; other big ones I've not been able to play due to lack of funds for the longest time unfortunately! I've only listed ones that are out currently that I liked. There's a lot of cool stuff coming later this year, like Super Mario Maker II, Astral Chain (by PlatinumGames, the makers of Bayonetta), Luigi's Mansion 3, Animal Crossing, Fire Emblem: Three Houses and much more to keep our eyes on