Science projects are great for my (then) three year old, but every now and then we enjoy simply just playing with some toys. Here’s a video of a really great toy (I think purchased on PChome  ) that should provoke a little bit of the tinkerer in him.
This toy is neat in that it can be taken apart and put back together again relatively easily. The plastic wheels and rubber treads can be removed (they did come slightly loose after a few iterations), and the exterior plastic casing can also be opened up for easy inspection of the battery housing. This Excavator Robot utilizes sensors and is mostly successful in staying on the wide black marker lines. What looks like light sensors are at the bottom front nearby the motorized direction controlling forward wheels. The sensors do work repeatedly, although not always so at corners. In the future we should do an experiment checking the optimal width of lines that the robot can follow. Compared with visual sensors, I wonder if there are robots that follow conducting paint inductively, although this might not be the best idea for a children’s toy.
For my children, a good way of systematizing some of the explorations into robotics and toys, is using powerful code or software that can interface with the hardware. (For when they’re a lot older) Programs such as Mathematica and Matlab, known for their number crunching capabilities, can also be used for interfacing with robotic control systems (see for example these great blogs, Christopher Wolfram  and Cleve Moler ).