The Playtime Audio Blipblox might fit the bill. It's a proper synth, but it's built like a Fisher Price toy.
https://www.youtube.com/watch?v=1s98te0b4bo
From a more music-oriented perspective, I might suggest the Casio SA-77 - it's a whole bunch of fun for $50.
https://www.amazon.com/Casio-SA76-mini-Sized-Tones/dp/B00Q83...