A basic set is a great first birthday or Christmas present when they're too young to really appreciate much and you're better off getting something they'll learn to love. A classic figure of eight is enough when their little, and as they grow, so does the game. Expansion packs open the possibility for more and more intricate tracks (for parents as much as children). But the real fun comes with the add ons;
Collapsing bridges (a personal favourite),
You and everyone else can add to the set up so that the complexity of the game grows with the child.
The official merchandise is wonderfully well made but can be a little pricy. It's well worth getting a small extra of proper Brio rather than something more spectacular that may not fit with the set properly.