Others have commented on homebrew, but there are a couple "game engines" available for the Switch that don't require modding. Nintendo released their own, Game Maker Garage, which uses a block-based coding environment. There are also two I know of that use a more traditional programming language: SmileBASIC and Fuze.
Both are similar to PICO-8 or other "fantasy consoles" where you get a full environment that runs on-device, with a code editor, tools for editing sprites, tile maps, music, and sounds, and a library of stock assets. Fuze also has a cheap "player" app so your kid's friends wouldn't need to buy the full app to play anything she makes. I've only poked around them a bit, but they seem like pretty capable environments that might be a good option for a kid that's interested in programming. The obvious limitation is that you're locked in to that platform - as far as I can tell there's no way to run your games on PC.
Both are similar to PICO-8 or other "fantasy consoles" where you get a full environment that runs on-device, with a code editor, tools for editing sprites, tile maps, music, and sounds, and a library of stock assets. Fuze also has a cheap "player" app so your kid's friends wouldn't need to buy the full app to play anything she makes. I've only poked around them a bit, but they seem like pretty capable environments that might be a good option for a kid that's interested in programming. The obvious limitation is that you're locked in to that platform - as far as I can tell there's no way to run your games on PC.