I would stay away from anything so complicated that you only can use it as a black box, like in the video a USB interface. Instead I would use very basic components, preferably things which generate a sound or display something, these will appeal to your youngest too.  
One possible project is an electronic die. You can do this with a few HCMOS ICs, like a 7-segment LED driver. It has the advantage that you don't have to introduce transistors yet (transistors are Really Complicated Devices!!) and they *see* something happen.