Croc: Legend of the Gobbos was a childhood favourite of mine that I played when I was 6 in the mid-late 00s and the remaster just came out yesterday! It’s a fun 3D platformer from the 90s that was on the PS1/PC and was mainly made for kids. On PC it’s a GOG exclusive though, so you’d have to set up Heroic Games Launcher to download the game, then add the game manually to your Steam library if you don’t want your child messing around with desktop mode. I think it’s very suitable for his age. The original game had tank controls which he might not like if he’s used to modern game controls, but the new remaster has the option for full dual analogue stick controls which makes it feel like a normal modern game to control. I run a Linux desktop and the game works perfectly on the newest Proton version.
It’s alright, those games are all Steam Deck verified so they’ll play just fine.