Godot's download size is pretty small, and there's a small demo included, along with a few other demos that are similarly pretty small files that you can downlo

Godot's download size is pretty small, and there's a small demo included, along with a few other demos that are similarly pretty small files that you can download directly from the client, mostly MIT-licensed, including a first-person shooter demo, I think. Grab some freely-licensed assets (there are a bunch of them, they're pretty easy to find with the obvious search queries, mostly CC-licensed), then the code for what you want should be pretty easy to hack up in a couple hours. GDScript, while not very elegant, is simple enough to where you should be able to learn it pretty quickly (it's aimed at non-technical people), and if you don't want to use it, you can use other languages, too. Some of the ways it does things is a bit confusing, but if you keep docs and a search engine close-by, you'll figure it out; good luck, it's always nice to see parents who care so much!