If you’re looking at teaching robotics, a line follower is one of the simpler robots that you can make that will still do something interesting. With some relatively simple sensors, a robot can adjust its course to go left, right or straight, and can be powered by a microcontroller, or even by discrete components.
For a great example of what these robots can do, check out the race/course video below. It features some interesting obstacles that look quite challenging to the participants.
On the other hand, relatively advanced robots like that aren’t what you’d challenge someone with as an introduction to electronics; perhaps something simpler would suffice. If you’d like to purchase a kit, rather than source all the components yourself, then the jolliBot line follower — which requires you to supply a readily available Arduino Nano — is worth a look. Besides basic line following, the robot can provide its own illumination, and can be programmed for PID control as shown at 3:00 in the video below.
If this isn’t quite what you’re looking for, be sure to check out this blog post which compares several different robotics kits and electronics modules.