I built this 13-key (pure analog) music synthesizer using a chain of 13 astable multivibrator circuits whose outputs are connected to an audio amplifier chip (LM386) and 8-ohm/0.2W/2? speaker, all powered off a 9V DC battery.

Check out how it sounds! (YouTube demo - safe to start at 0:49s to skip the explanation of how it works - Wadsworth's constant ;) )
https://youtu.be/peobYmtDthQ

Each individual circuit is tuned to one of the 13 frequencies in a musical octave (C5, C#, D, etc. up to C6) by varying a fine-tune trimpot that is in series with specific resistor values and which get the oscillation into the ballpark frequency.

You can read more about the challenges of pure analog designs, and alternative digital designs in the article.
http://mathscitech.org/articles/building-music-synthesizer

For a gallery of electronics projects that (or your children) can get started on, see:
http://mathscitech.org/articles/exploring-electronics

Electronics Supply page here:
http://mathscitech.org/articles/electronics