OpenHoerLevin is a open and free mp3 player for kids. Follow the idea of Hoerbert (https://de-de.hoerbert.com/) build with wood and some electronic scrap.
Some cheap hardware from china:
Attention: Most hardware shipped without any technical documentation. I could find specs on the internet but similar products aren't compatible. Of course the project is also possible with other Arduino boards and modules but some specs could be different. The IoT WIFI Nano Board came from another project and isn't the cheapest board, but with the Wifi it is open for next freaking feature - we will see.
Looking for more? Thomas also made his own "Hörbert": http://chrigas.blogspot.ch/2014/10/arduino-mp3-player-1-auswahl-der.html
Based on a mp3 kit: https://www.elv.ch/mp3-sound-modul-msm-2-komplettbausatz.html