Comment a quality calculator? (Score 1) 368
A decent quality scientific calculator and enough training with it that they can start using to discover the joys of solving problems. I think I had my first solar powered scientific calc when I was about ten. A handful of years later in high school I moved up to a more complicated graphing model. I'm sad to say I don't have the original calculator but I still have the latter. It served me well through high school, college admissions exams, and then a bachelors degree in the sciences. Of course these days I write boring but comparatively lucrative line of business web apps, so the most complicated math I ever have reason to do can be done with gcalctool in simple mode.
I won't get into the emacs-vs-vi wars of TI/HP/whatever, but get them a solid useful tool that can be used for most any science endeavor and you'll have done them a favor, imho.
(And yeah this might not be appropriate for the 7 year old, but I'd wager the 9 year old is mature enough or on the cusp of it...)