The electronics in the Thingamaplush are based on the Thingamakit design, available on Bleep Labs' website. I tweaked the design a bit to fit my own purposes, ending up with a sort of hybrid between the Thingamakit and the Thingamagoop. The electronics are stuffed into a plush robot I designed myself, and assembled with the help of my mother (how's that for some geeky mother-son bonding?)
This instructable will detail sewing the robot body, and stuffing it with my custom made board. Of course, you may remix any part of it to suit your own needs. You could design a different body, or install different electronics. Maybe an Atari Punk Console? Or a Robot Voice Modulator? It's up to you! You could even omit the electronics completely, to create a cute little robot toy.
EDIT: New videos added! See Step 11...
Please note that I made every effort to make my design safe for a toddler to use, but I can't guarantee its safety. If you're a parent you know how hard kids are on their toys - they are dropped, sat on, stepped on, chewed on, drooled on, and generally abused in ways that the Underwriters Laboratory can't even imagine. With this in mind, observe your child when they are playing with their Thingamaplush and make sure they remain safe. Unsupervised, I'd say the minimum age is 6.