Well, if you are a gamer yourself, not just getting trend games or classics, then you might try to get them into what you like (as long as not nightmare inducing).
When I was three, my dad got me into Command & Conquer - Red Alert and then later, whatever console we were able to get. Had an old nintendo, so classic mario bros and then a ps2 with snowboarding, atv, rugrats, the incredibles, lord of the rings.
Though, since you said that they are starting to read, then perhaps they might like typing games too such as Epistory - Typing Chronicles.
Since enjoy Slime Rancher, maybe Minecraft and Satisfactory would be nice. Exploration, building, patience, following instructions.
Though, it looks like they enjoy Nintendo games and those usually be all ages. You might like to also try Spyro and Crash Bandicoot from the same area of games.
Also, World of Goo is always fun.
side note: on the old style NES Classic, I quite like the game Edge of Hearts. ..has some reading, but is mostly hack and slash.
When in doubt, LEGO Games win them all. ...Though, you could also get them to look through games too and choose.