It is hard to beat these four Robotis Kits.
Play 600 kit.
Great intro to robotics kit for kids. I have seen kids ages 4 and 5 years old have no problem building robots with this kit. There are instructions on how to build 7 different robots with this kit.
Play 700 kit.
A great intro to robotics kits for older kids. All of the parts in this kit work with the Play 600. So if your kids like the 600 then buy this kit when they get older as a upgrade. This kit adds sensors. remote control and programming.
This is a great intro to humanoid robotics kit. Better for older kids in high school and up.
This kit is for advance robotic users. It is the lowest cost robotic platform for learning and using ROS.