I feel like its probably mostly american kids nowadays that would get bored with TOS right away....for instance, I tried showing my nephew (who is 5) an episode of TOS, and he lost interest pretty quickly...
I showed him the new movie and he loved it. I plan on starting him off with specific TNG, DS9, and VOY episodes, but I have to find episodes that have sufficient comedy and LOTS of action. Im thinking lots of klingons....
The thing is, nowadays, american kids have more than one entertainment source, if they dont like what you are watching, they can go watch tv in the other room...too much stimulus...
Back when I was a kid, it was star trek or nothing, and that allowed me to focus on the show and realize that it was awesome...(although I think as soon as I heard the intro music I was a fan)
Yeah, I think the new movie has the right touch of action and repose for kids these days. I'd start with that, except you need to really hammer home the fact that it is set in the alternate universe. Once my niece got that (and she loved the new movie), she was really curious about the older shows and movies. I should mention that she *has* seen quite a few of the older series episodes as well as First Contact, Generations, and all of the TOS movies -- so she's pretty much a Trek veteran for a 10-year old.
But I agree that the new movie is a good intro in terms of generating interest in the younger kids.