Back in the year 2000 LEGO had a short-lived (6 sets) Mickey Mouse theme. The cheapest set in the list (at US$10) was 4164, Mickey's Fire Engine. At 26 pieces this was a simple build, but very appropriate for the age range (4-9) years who, at around 4, would be making the transition from Duplo to System (these are System). The Fireman Mickey fig is cute and easy for little hands to grab since it is larger than a normal minifig. Also, since it is all one piece, younger kids won't swallow parts. Of course the flip side for the AFOL is that you can't easily swap out the head or otherwise use the parts.
Buy it on BrickLink. See the inventory on Peeron and the instructions at lego.com. Some more pictures: