Ask HN: 9-yo son wants to build a game, I'm lost. What can I do?

Tell him the first step is to build a working game with 2% of all the features he wants.

Then you add features one by one. That helps with the accomplishment aspect. You guys may never reach 100%, but this is the most likely way to get there.

I think I'd recommend something like GameMaker. I've never used it myself, but it's a self contained IDE that has everything to make decent 2D games.

I'd advise against the pitfall that 3D games lure you into. That includes engines that are 3D with a 2D "mode". There is so much leaky abstraction, and the doco will be intermingled with 3D all over the place, leading to frustration and unnecessary complexity.

If a game is fun in 3D, chances are you could write a game in the same vein in 2D and carry the idea and execution across. 3D games are beyond ridiculously hard to execute on by oneself (well), and require exponentially more work to complete.

There is no gain, in terms of discovering the joy of making games, in diving in 3D first.

If you prefer a more programming centric approach, with no IDE, I recommend Love2D. It's a joy to use, well documented.

If you want a full blown low level approach, I recommend Raylib. It's fantastic and fun to use as well. But at this point, you're swimming in the deep end.

Also worthy of note is PICO-8. It's a fantasy console that can be easily ported on 'open' console (such as the ambernic), making your game feel alive, being played on a hand-held console. But as for the learning experience, I found it to be more of an expert's getaway retreat, rather than a good first choice for an engine. That said there is enough material out there and small enough API, to pick it up rather quickly, all batteries included.

The way to use Venn diagrams is to distill "the game" (which, for any pre-teen, and for most first-time adults too, is going to be a jumble of features and assets) into its absolute core: the big picture, philosophy, principles, themes, "design pillars". The core is the overlap of all the specific stuff: when you implement something, it has code, it has visuals, it has audio, and it should all relate to the core. So, Tetris, Sokoban, and Minecraft are all about "playing with blocks" at some level, but they differ in that the core also contains some other aspect of play. Every piece of media that works for its audience - they find it believable and engaging - is representable in the diagram.

So when your son asks for this or that feature, have him work through it on a diagram, filling in "what sound does it make" and similar kinds of details while you address the big picture parts that are hardest. Make lots of little diagrams. You'll eventually hit on one where you can reasonably hope to prototype it with your chosen tool. That's good enough, when you are 9 years old.

The key is to reduce the barrier to entry and get something emergent to happen. Like a zombie shooter where a shot zombie stumbles into a fire barrel that adds "fire" to anything it touches. This could create a flaming zombie that spreads fire to the other zombies. Boom, the kid is hooked and before you know it you're paying for expensive and dubious game design curriculum cursing my existence.