VEX is an excellent introduction to robotics. And is one of the most affordable robotics programs available.
VEX Robotics Competition (VRC)
If you remember erector sets, VRC is similar. It is mainly geared towards middle school and high school (some students participate in college). With VEX, students will get a chance to learn about and use different types of sensors, wheels, motors, gears, etc. For programming, they use RobotC or EasyC.
Learn more about VRC here: http://www.vexrobotics.com/vex
VEX IQ
This VEX program is geared towards elementary and middle school students. VEX IQ uses plastic pieces that snap together.
Learn more about VEX IQ here: http://www.vexrobotics.com/vexiq