What's your kid interested in?
I mean...Persona 5 Strikers comes out today
There are entirely too many games on Switch and the Switch eShop is useless in terms of figuring out if a game is good...always do a quick Google search for a game + Switch reviews to see if the Switch version is worth anything or not. Assumptions should be easier to make than they are here...some games run way better than they have any business doing even in handheld, while others were clearly not made with the Switch in mind and are just shoved out to fulfil a checkbox. Bethesda's Doom games run way better than they should (though obviously still not the best way to play them), but Obsidian's The Outer Worlds is a cruel joke on the Switch (if you ever played Wolfenstein 3D or Doom on the SNES...or can imagine what those games would be like on the SNES...it's kinda like that).
Are you expecting to play mostly docked (TV) or handheld? The regular Switch that can be bought in stores has a significantly better battery than they used to, even better than the Mini's, so that makes figuring out which model to buy much easier. Buying a used one is tricky because of this, but if the Switch box is mostly red it has the newer battery (the change happened long enough ago that stock would have sold through a while ago). Newer limited edition versions, such as the Animal Crossing one I have, also have the upgraded battery.
Joy-cons still drift and Nintendo is still shitty about it, but you can at least buy individual ones now so that you don't end up with 1 working left joy-con and 4 working right ones like I have. If you plan on playing it docked for any amount of time at all I'd consider a Switch Pro controller an absolute must.
Games...man, too many. Too damn many. The Switch has taken the role of multiple competing platforms of old and as such the games library is huge and varied...you have your weirdo T & M-rated Japanese-as-f*** games that would only make it across the ocean on the PSP or Vita before it, you get your mid-sized Nintendo handheld-ish games (which creates some real pricing headaches as Nintendo loves charging $60 for ports of old games that cost $40 years ago), you have your world-class indie games, your iPhone/Google Play-level trash indies made by one person in an afternoon, you have your Wii-esque shovelware (including re-releases of Wii shovelware), your big tent Nintendo AAA games,
and multiplatform third party releases (which still weird me out seeing on a Nintendo platform, though these will likely get more rare as the hardware gap intensifies). There's a good amount of something for everyone on the Switch, basically.
I'm expecting/hoping for a glut of Atlus releases on the Switch moving forward, just in case there weren't enough massive time-sinks already on the platform as-is.
Hades was my GOTY among games I finished/wasn't a remake and comes with the highest of recommendations, even if you hate rogue-likes...actually especially if you hate those kinds of games. One of those games where every aspect of the studio was clicking at nearly every step...the story is interesting and doled out in a tantalizing way, the gameplay is satisfying even if you're a button-masher, the game balance is as good as I've ever played, the art is just insanely good, voice acting is ace...there was nothing about a rogue-like hack-n-slash game set in Greek mythology that should have appealed to me, and yet...just be warned that Greek mythology without going through the Disneyification sterilization means this game earns its T rating.
Since you seem to be enjoying P4G I'll say keep an eye on Tokyo Mirage Sessions #FE Encore going on sale sometime. This is a real hard one to sell to people because it is the most Japanese-ass JRPG I can fathom and it's hard to separate it from not being what people wanted when they heard about a Fire Emblem/Shin Megami Tensei crossover (Persona is a spin-off of SMT)...but it has one of my top 3 turn-based battle systems ever and the Tokyo idol setting makes for a good change-of-pace game if you need a break from a more serious game. This is a really weird game to try to sell people on, especially as it's more of a idol-themed SMT spin-off with some familiar Fire Emblem names thrown in...but if you're enjoying P4G and you end up liking a certain character you'll be meeting soon there, you'll probably like this game.
Fire Emblem: Three Houses is a must. Tons of game there, too.
Unfortunately the Switch has not reversed the trend of Nintendo being the worst platform for sports games...but with the condition most sports games are in right now, you're really not missing much unless you're addicted to throwing money away on virtual card packs.