I'll also highly recommend GBG. My eldest had a good time building very creative things when he was about 9, and my younger son who is 6 has built a 3d platformer. i haven't a clue how to program it, but they pick it up easily! Also, they both love Scratch (not Jr) on a browser. Not 3D, but a great outlet for their ideas.
If it's just level building, then Super Mario Maker 2 (also Switch) and Levelhead (Steam, more accessible on Switch). There's also BQM (Steam,Switch) for isometric dungeon designing.
If it's just level building, then Super Mario Maker 2 (also Switch) and Levelhead (Steam, more accessible on Switch). There's also BQM (Steam,Switch) for isometric dungeon designing.