A kid knows what she wants but not what the price that she will have to pay for getting it. Today they are interested in something, tomorrow something else, specially if it is hard work, like 3D. Young children have not the mental abilities to endure hard work. They should be playing and having fun.

When they grow up and have friends and are happy in a healthy enviroment then they can endure healthy hard work later.

Simplicity and immediate feedback is the most important thing for a kid. Forget Unity, Unreal or other environments that require at least a 14 year old and complex rational skills.

I volunteer teaching young children things like 3d printers, mill machines(they can not access dangerous machines until they grow up, but they can design things)... Most children are just going there so they play with their friends. 1 in ten will become an engineer.

I will just use Pico8 or Scratch to focus on the design of the game and SIMPLICITY. Teach him basic skills like writing and drawing and basic music and sound, and basic programming so they can introduce real life asserts there, in 2D..and specially game design, to design fun games prototypes.

Start doing something basic that is funny in 2D. If he does not loose interest, raise the bar. Since and repeat.

This is like if he wants to be an Orchestra director. You start teaching him the musical notes, having fun with melodies, then harmony...

Games are designed with minimal proof of concepts, that have no textures, sound recorded with the phone, with a basic dynamic of the game. You test that your basic prototype is fun. Only then you add textures and better sounds, or sophistication.

3D modelling, inverse kinematics and Mocap, painting and assigning textures in 3D, it is just so hard and little reward for a kid, with long periods of delayed gratification.