I recently saw a presentation about GreenFoot (a java based learning environment for children). It looked awesome. If I would have kids, I would give it a try
Link to the presentation
It is a very playful environment, where you could start with very basic methods. The kids learn thinking in an object oriented way (you cannot instantiate an animal, but you can instantiate a cat). And the better they get, the more of Java you can uncover for/with them.
