Recent versions of Unity are actually using Roslyn ¹) but they are admittedly running a bit behind on C# language version. The currently supported version is at 9 while 14 came out last month. It's not really a huge issue in practice, though.
With Godot 4, the big difference between Godot and Godot .NET is that the version with NET support does not build to web and mobile support is 'experimental' ²). Also, they are two completely separate downloads and editor binaries, which makes switching languages decidedly non-trivial.
For a 2D game, if you can live without building to web, I'd pick Godot. Otherwise, I'd pick Unity.
This may be a dumb question, but I couldn't figure it out from the website: Does this app allow me to actually view my photos? Can I double-click a filename? Can I get a page of thumbnails? Some sort of a gallery view?
You may want to list clearly somewhere which types of digital asset you support, because different industries have different ideas about that.
This only handles images, correct? And not video, audio, Office documents, PDFs, 3D models and animations, CAD/CAM drawings, PCB layouts, web pages, or code?
The Dutch language quote as displayed: "We mogen niet uit nonchalance fouten in een programma aanbrengen. Dat moeten we systematisch en met zorg doen.".
Feel free to run that through your favorite translator.
The subtitles: "We should not introduce errors through sloppiness but systematically keep them out."
The translator missed a very dry and very Dijkstra joke.
As others have also mentioned, the most important thing you can do is show interest and engage with your son and his ideas. And that really requires only your time and maybe a pencil and a bit of paper.
Figure out what kind of game this is and what part of the design process he's most interested in. Maybe it's not so much the game but he just wants to design cool game characters and have them walk around?
If you both do want to make a computer game, I think the various Make-A-Game games, many of which have already been mentioned, are your best bet. Game Builder Garage or Super Mario Maker (Switch), Wonderbox: The Adventure Maker (Apple), RPG in a Box or Super Dungeon Maker (PC), Roblox, whichever is the best match to your son's ideas.
Unlike some others here, I would recommend against switching to 2D. From a didactic perspective it absolutely would be the way to go as it makes a great many things a lot more manageable, but from experience you run a big risk of losing engagement and interest, especially as your son explicitly wants 3D. For a lot of kids in that age group, 2D just isn't cool.
I'd also recommend against full-blown development environments like Godot or Unity, at least until he's just a bit older. When he gets to that point, Unity does have some neat templates made especially for kids, where they get kind of a starter game and a walk-through on how to build it out.
Ardour is GPLv2 open-source, but they still do somewhat pointedly attempt to dissuade you from building from source ¹).
Now, I fully understand why. And I think charging for pre-built binaries is a totally valid way to attempt to finance an open-source project. The amount they're asking certainly is a pittance compared to the commercial offerings.
With Godot 4, the big difference between Godot and Godot .NET is that the version with NET support does not build to web and mobile support is 'experimental' ²). Also, they are two completely separate downloads and editor binaries, which makes switching languages decidedly non-trivial.
For a 2D game, if you can live without building to web, I'd pick Godot. Otherwise, I'd pick Unity.
¹) https://docs.unity3d.com/6000.2/Documentation/Manual/csharp-...
²) https://docs.godotengine.org/en/latest/tutorials/scripting/c...