My biggest mistake is always feeding them with material that is not appropriate for their age yet.
Like RC airplanes that are even difficult for me to flight.
Bat what works best so far is:
1./ No television available at the house
2./ Limited, and directed/restricted access to computer, unless they are programming Lego NXT, RubyKids, or practicing vim skills. (yes, the 9yo edits in vim)
3./ Plenty of reading and art material available.
Most of the time they are either building, reading, or painting something.