So, in my research for Art of Atari, I was able to acquire this prototype Atari 5200 Kids Controller from a former Atari employee who had it stored away. It even has some of the overlays, as you can see. I don't think anyone else actually has the 5200 KC overlays that I'm aware of.
I know the one seen at CGE (supposedly the only one at that point) was reportedly non-functional. I'm not sure about this one, as I haven't yet opened it up, and also have my 5200 in storage for the moment, so I can't test it at the moment. But I will take a look at some point soon.
It's a super cool piece of Atari history, and I'm excited to share it now with all of you!