During Christmas dinner yesterday, I was talking with Craig McNeil about preparing myself for helping Henry & Owen to explore building and controlling their own devices. Part of Craig's research is building scientific instruments and so he is well along the hardware path whereas I am further along the software path. Here is a list of resources that came out of the conversation.
Modern Device has a Arduino microcontroller kit originally designed by Paul Badger for his RISD students's art projects. It is very reasonably priced and easy to construct and program using Processing.
Picaxe is another microcontroller with lots of sensor accessories. Craig uses this in his instruments.
Squeak is a modern development of the SmallTalk (textual) programming language. The most significant development in this for kids is EToys. Squeak by Example is a good place to start learning Squeak.
EToys is a kid orientated (graphical) programming language. The One Laptop Per Child EToys site has a good collection of information about EToys beyond how to use it on the OLPC.
Logo was the first software language designed for children. I think this is still the language used in Lego Mindstorms. This is today overseen by the Logo Foundation.
NetLogo is a simulation orientated variation of Logo.
FIRST is an organization that promotes interest in the sciences. I know about them because at the last Providence Geeks gathering there was a presentation about RI's involvement with FIRST.