The HEXBUG robots are pretty cute and occasionally give the impression of sentience. With a little bit of work you can upgrade the bug brain with a microcontroller.
The HEXBUG Nano is basically a pre-built and well refined bristlebot clone but unlike the bristlebot it is able to flip itself back over to avoid getting stuck. It does not appear to have any sensors or control and relies on random emergent behaviors to give the impression of intelligence. But, if someone gets me one as a present [hint] then I'll try to see if there is a way to steer them and maybe add some light sensors. Though I'm not sure how you would work out the differential equations to describe the motion induced by the oscillating weight attached to the motor.
The HEXBUG Nano Habitat Set looks like the robotic equivalent of an ant farm and could be used as evidence in your trial after our future robot overloads take control.