This is one of those things a friend tells you about and then you don't believe them... But then you find out that this is actually a thing and it is actually built for your child!!
The Blipblox is a fully functioning synthesizer for your child, it even has a MIDI in port on it. The basic architecture of the synth is laid out on the box itself so if your child is discerning enough they may well be able to figure out exactly what is going on. Or once then get a bit older they can start to learn the box.
So is it a toy? or is it a synthesizer?... You can decide.
More info at Blipblox