More than 25 years ago I was struggeling with an old SHARP MZ80-K computer to teach myself assembly language and to start tiny automation projects using its parallel port.
Today with LEGO Mindstorm things are much more easy, but also awfuly expensive!
I just found a new startup company (located very close to where I grew up) who provide open source hardware and software easy to use for fair prices.
http://www.tinkerforge.com/
Just connect actuators and sensors to small boards, connect everything via USB to your computer (or mobile phone) and program it from there in lots of different programing languages like C/C++, C#, Java or Python.
Buidling a small robot is a question of a few hours and not a complete diploma thesis. It is not that robust as LEGO Mindstorm, but you get access to the real hardware and therefore giving you a chance of understanding how things work.
Here is a small video showing everything:
So if you want your somewhat older kids to learn about electronics and emebdded programing this seems to be a highly recommended gift for christmas !