Let me address that last point of project scoping: it's a great skill to teach, and the right way to do it is to introduce Venn diagramming. This is a serious tool for any kind of creative project.
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 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.