That is how it went for my sons as well. My older boy (Dominic) has been obsessed with trains from 2 years on. It has turned into a hobby for all of us. Dominic, though a little more camera shy, was the brains behind this layout. The height (2ft), the reach distance (less than 2 ft in case they come off), and the number of independant lines (two loops, a railyard, and a factory spur) were all his specifications. We used Atlas software to lay it out, I did the computer operating, but he guided me based on what he wanted. I was never into model railroads, but now that we have done this I think it is pretty cool.
http://www.youtube.com/watch?v=dbDJDfI7wVE