Here is an open source Arduino kit designed to teach your kids a thing or two about coding. Snapino lets you prototype circuit designs in a flash. It can be used with Arduino compatible shields and components for more complex projects. You can use this kit to complete night light, distance sensor, click counter, and other fun projects.
The kit ships with 14 parts for you to get started, including 6 Snap Circuit modules. You will need a 9V battery to power it. This STEM kit is available for under $40 at this time.