Then, even if the language would be different, they can transfer to a proper game engine like Unreal.

But publishing the game in Roblox for real could still be a good thing (or not). Just don't expect to "get rich fast". It can teach that even you have a cool game, nobody is gonna find it and you'd have to stand out. In the end it's a bit like YouTube. Same exploitative relationship. You might make money. Most won't. Many will feel lots of pressure for no reward. And even with reward the pressure might become too much.

With Roblox he will definitely be able to just use pre-made (of not super awesome looking) cars and zombies. In fact the Roblox tutorials come with a pretty good looking Zombie actually. With animations and all. They also have some official item collections which included high and low poly stuff.

You can help him put together a simple version in a couple days tops probably. Like something that's literally just car that won't drive, some zombies that walk towards the player location if in range, he has to long press to "repair" the car and escape. First version done.

Then expand from there, like have an actual "broken car" model vs "working car". Start adding houses and streets and an "escape zone" you gotta reach etc.

That said, don't underestimate his curiosity. I thought my kids wouldn't get past that either and now I sometimes glimpse or they show me what they've been working on. I just showed them some 3D software like Blender, they got a tablet and a cheap graphics tablet for the PC. And I do see them doodling around with it, trying things. Just like we did. But with cooler tech. Just today actually I told one of them when they were on the tablet how I watched Star Trek as a kid and always wanted to "live long enough that I'd have something like the P.A.D.D.". And look here we've had iPads for "forever" now and the kids think it's normal to have.

* https://kenney.nl/assets

* https://kenney.nl/starter-kits

The asset range includes 2D & 3D and in a variety of graphical fidelity/style from 1-bit to low-poly--including sets which are designed to work together.

The "Starter Kits" are a more recent development which aim to get you something working out of the box with Godot which you can then customise: https://github.com/KenneyNL/Starter-Kit-3D-Platformer

I'm pretty sure you'll find both cars and zombies to start with... :)

(Another person who shares high-quality free assets worth checking out is Kay Lousberg: https://kaylousberg.itch.io )