@spheric_el you, sir, are a genius! I totally forgot I have a gen1 Kaossilator in little girl friendly pink sitting unloved next to my guitar! A quick check and itll be good to go!!
A looper.
I’ve been watching videos on loop artist. On Sunday, I was mesmerized by Ed Sheeran and since then I have been watching a wealth of vids on YouTube.
The thing is, and since I see a class full of pre-schoolers on a daily basis, it has to be fun. I can’t stress that enough. It HAS TO be fun. I think a looper and mic (which is probably lying on the opposite side of your Kaossilator lol) will bring copious amounts of fun.
Down the road sometime they may even feel like they want to add more arsenal to the input and start trying out your instruments.
It’ll probably be heaps of fun for you too. I can almost see you and your kids having a cracking Christmas Day with a looper! 
It’ll be a right laugh!
Once your kiddo is older, I would recommend the LittleBits Synth Kit and other kits. Definitely a way to learn some electronics and critical thinking skills.
Little bits are def on my list for later. Don’t think I could justify the money to the missus ATM 
The twisted electrons Cheshire synth is definitely going to be the intro to soldering later!!
Edit: oooo https://youtu.be/nt6CW3oEItk
Yeah, I’d bought it on eBay for about £15 to use in a rock band guitar… http://www.instructables.com/id/Make-a-Korg-Kaossilator-Guitar/
Never happened in the end. So at least it will get used
How about elektrosluch? Not exactly musical, but if kids are a little bit creative it can be a lot of fun.
You can make it DIY as well if you are into hand made gifts or want an easy project.