I'll just give an age range for reference: 6-8

Ones I hear about the most:

• Pokemon Black
• Pokemon Mystery Dungeon (I forget which but I think it's a DS game- not a Pokemon person myself)
• Nintendogs (Golden Retriever edition if it matters)
• Mario Kart DS

Ones his parents have said he likes:

• Sonic Advance 1&2
• Most of the Super Mario Advance series (his mother loves Yoshi's Island and I think that's the one he watched her play sometimes)
• Mario Party (the N64 ones)

Ones he's told me he dislikes:

• Fire Emblem (one of the GBA ones)
• Harvest Moon: Friends of Mineral Town.

I'm waiting until he's a bit older to push for some games (notably the Metroid series) as I think he's still a bit too young to get them. Pokemon seems to be a huge hit (he's getting my switch fairly soon which I'll get him whichever of Sword/Shield and Violet/Scarlet he wants if he's good at school) and he'll have access to my copy of Mario Kart 8 deluxe then too.

One of my "goals" with this (I say this loosely cause I just want the kid happy) was to try to get him an appreciation for older games too so that he just has more options available to him instead of strictly always chasing the newest thing, and I also know his mom has plenty of games that she probably hasn't played since her childhood that she'd probably love to revisit in the context of playing with her kid.