Very nice! I built a similar system with my young kids a few years ago called Qrocodile [1], which used a RPi (inside a custom Lego model) to control our Sonos system by reading QR codes printed on small cards. QR worked well because they're cheap to print. We printed a couple hundred album/song cards (each with album art) and a number of control cards. Fun project. Source code and all the instructions (for server, client, and card generator) are in the GitHub project [2].

[1] https://labonnesoupe.org/2018/02/14/introducing-qrocodile/ [2] https://github.com/chrispcampbell/qrocodile