This is the source for my self-made copy of the "Hörbert", a simple MP3 player for kids. The project is described here:
https://www.florian-wolters.de/blog/2015/12/29/hackbert-the-audio-player/
I made small changes to the source code and wrote a simple shell script that allows for preparing the SD card. It also converts the input files to WAV format with a single channel and 22.050 kHz sample rate. That should lower the current drain compared to playing MP3 files.