I think since the kid is interested in building a game that they designed themselves, it's better to handover a (simple) game engine to him. A game engine enables way more possibilities than modding. You can add multiplayer over websocket/webrtc, you can write your own shader, you can write your game logic however you want etc. All of that without the overhead/limitation of a modding environment.

Also, out in the wild the job opportunity for Garry's Mod developers is very scarce, if any. Just something to think about if op wants to do some carrier development for the kid while having fun.