I've built my kids an instrument that cannot be played badly. Any key, any two keys, any random bashing... it always sounds intentional: intonation, pentatonic by default. When you shake it, it plays back the notes my kid just played. When you tilt it it changes the vibe. It's called "grajek" Polish for a little fiddler, the kind who plays at a village wedding. Hard to translate, but easy to hold for the little hands. Under the toy there's a "serious" engine: 11-limit just intonation in the spirit of Harry Partch, sympathetic string resonators, an echo tape that ages every pass, and no BPM anywhere (the pulse locks to your playing). Whole thing done on @M5Stack Cardputer-ADV, ~$60 off the shelf.