I built this very simple musical toy for my son a while ago, before the lockdown. I used it again while we were locked in. It is basically a python script that reads the input of DanceDanceRevolution compatible USB mat and plays sounds for each "key".
I did an animal themed mat, but pick any theme you like, it will get annoying at the end when your kids play with it ;-)
The initial plan was to run this script on my Raspberry Pi 3, but the python library I use doesn't seem to like the Raspbian sound server. As the script works perfectly on my Linux laptop, I didn't investigate further.