I would recommend several things,

Squeak by Example[1], or better yet etoys[2]

How to design worlds, as already mentioned here

Alice[3], with something like [4].

Good old scratch from the logo tradition would also be cool[5].

[1] http://squeakbyexample.org/

[2] http://www.squeakland.org/

[3] http://www.alice.org/

[4] http://www.aliceprogramming.net/

[5] http://scratch.mit.edu/

Check them all out and decide for yourself. All of them are very consistent and fun.