Pokemon

Palworld

/s

Depends on how young and what kind of post game content you are looking for. Either way the more modern games have more post game content though, some even with dlc.

If very young, the let's go games are probably a better choice as they are more kid friendly. Story follows red/blue but has modern elements added to it and post game is basically catch em all.

Normal like games I'd say sword and shield or scarlet and violet would be better since they are in the switch, swsh I remember spending a lot of time in the post game for dlc 2 (crown tundra) because it had a cave you could take with interesting raids you can do with randos on the internet and collect legendaries, SV had less interesting post game but is still active and can keep someone entertained for longer if they follow the events and releases. I'd be more likely to recommend SV if they have a switch 2 though, as it runs kind of bad on switch 1.

If a bit older, Legends Arceus is a lot of fun but requires a bit more skill and movement, has some fun new takes on the pokemon series and the catch em all has a different variety than usual games. Also Legends ZA will come out soon and will follow some of the same gameplay style as Legends Arceus.