First I want to point out how awesome it is of you to follow through on a fun bet like that. I mean, an expensive one yeah, but that's super chill to follow through on.
Preference of what the kids enjoy most + what their friends are all in on will be your best answer.
The second best answer is go straight to getting a PS5, but only if you discover that your kid does not have a preference.
Demon's Souls (ps3).
It showed me that games that aren't afraid to rip you apart from the inside out, aren't afraid to exist today.
Hence the soulsbornekiro colloquialism that now includes Elden Ring and the like.
Personally I always loved devastatingly difficult games that had some serious weight and gravity behind their concepts. Which is why I fell in love with Everquest (1999-2003), King's Field (ps1), Ninja Gaiden Black (Xbox), Devil May Cry (various consoles, entire series except for #2), the souls' series, Sekiro, Bloodborne, Elden Ring and so on.
The remarkable impact was: I felt like my own tastes in gaming were shared by someone else other than just me. It made me feel good about gaming again.
Elden Ring. got the platinum trophy on PS5 and still wasn't satisfied so I went and did it all over again with the PS4 version afterwards. Still no idea if PS5 trophies auto-pop if you get them on ps4 first and then switch to the ps5 version as I got the PS5 one first then saw the PS4 list was still blank.
Solved that one, I did. Now it's 2 plat trophies for the same game. It's that good.
At least to me.
FF7 Rebirth. Hook it up to my veins, I can survive from that games sustenance alone.
Symphony of the Night + Rondo of Blood Castlevania collection. That's where you should go next. It is often not that expensive, but I do forget the normal price. I think I got it on sale for $10 a ways back.
Symphony is incredible and is what inspired Bloodstained (and was made by the same creator as Bloodstained). The music fucking rips in that game. Secrets galore as well.
Rarest game I ever owned was Panzer Dragoon Saga. I do not own it currently. Had to sell it a ways back for more important things, but it definitely hurt having to sell it.
in Infamous 1/2 if you go evil the entire environment looks really cool by the end of the game. It's all dark and bleak, gloomy everywhere and if you go all-good it ends up as a beautiful picturesque environment. Going evil makes the world look way cooler in those games. I always thought that was a neat idea. To make the world that you travel around in, itself, look vastly different for each given morality-path you go down.