Lots of kids like making games, and RPGMaker is a game engine that lets you make games with 0 programming knowledge, but also gives you ultimate freedom to modify the game with plug-ins that you can download or write yourself.
I bought RPGMaker MV with a bunch of assets through humble bundle 5 years ago for like $80, and learned a ton about software develpment by trying to learn and make changes to the engine.
I guess if your kids are really just into web dev then this is would be orthogonal, but under the hood MV and MZ use html/css/js, where I think MZ was mostly a huge refactoring of MV to be structured like your more typical node.js project, so this would still be teaching them things relevant to web dev.