Ta for the compliments.
Always keep in mind how long it took you to come around. I know you want better for your kids, but perhaps they need to take the same path you did.
Think a step further: why would they be shocked and horrified to find out you're an atheist? Partly because of the stigma attached to us by the religious, but ultimately they would be afraid for you. You reject God, so you're going to hell, that sort of thing. The most important thing when it comes out, and it will, is to let them know that you are not afraid yourself. (Another little fact which might help is the idea that not everyone who claims to be religious really is.)
At that, they'll either ask why not or argue with you. You'll then have a line of communication open, and it's up to you. Expect your husband to get involved, and make no attempt to make this happen behind his back. Just be open, and listen to them all.
I grew up religious too, as you might have read in another answer of mine. What first set me on the road to doubt was the fact that my father was an atheist. He didn't talk about it AT ALL, he just told me once and that was it. I wondered why, and later, I found my own reasons.
As for kids' books, I recommend the children's and young adult books of Terry Pratchett. A humanist himself, his books often encourage critical thinking despite having fantastical premises. A favourite of mine is The Wee Free Men. Later, perhaps they'd enjoy his other Discworld books.
Incidentally, I just watched Happy Feet and the anti-religion message in that is fairly obvious.