I'll second Geoff's suggestions of Phrogram (used to be KPL), and Alice.
My only other suggestion is Lego Mindstorms NXT. The NXT's programming language is drag-and-drop, is very easy to use, and can do some very complicated tasks once you learn it. Also young boys usually like seeing things move. :)
I've used Alice and NXTs with some young kids, and they've taken to it very well.
