I will disagree with some things. I do believe that it is a matter of trains being seen as childish with the current generation, and they refuse to see otherwise until they see them running themselves beyond the initial "train set" on the floor. This is opposed to the view that seems to be held here that they cannot afford it; rather, a mixture of possibly being seen as a social outcast, plus a poor introduction and/or overprotective parents tend to be what keeps them out.
I will second the idea of whimsy that most people see as a good idea, and recommend having some inexpensive tinplate around for amusement and awe factors in addition to what I expect will be postwar and MPC. On that same note, you can't go wrong with a mixture of classic accessories and cars that interact, some rudimentary (display like) scenery, and the capacity to run many (3+) trains at once. I dunno if those will be possible for you, but keeping some of the history in it for them might not be a bad idea.
