I don't think there is much I can add that has not already been said. I've been following your story also, and I find it heart-wrenching.
Initially, I was worried that you were putting undue pressure on your kids. I quickly reconsidered. You DID set a good example for them by standing up for your rights. You did the right thing by reminding them of how hard you've worked to be able to see them more, and in showing them the legal agreement. By taking the choice away from your daughter, who legally does not have one, the guilt she might feel now and in the future is also taken away. Your son is a tougher matter, but you did the right (even if somewhat painful) thing by making your feelings known to him in a sincere manner. Everyone respects and responds to honesty, and kids are no exception.
And the sooner you let your ex know that these manipulations aren't going to work, the better. It will hopefully save you a great deal of heartache and frustration down the road - especially since you are legally in the right.
I would caution you NOT to bash your ex-wife in any way. Your kids will be able to see her manipulations for themselves, even if it doesn't always seem like it to you. Bashing their mother will only hurt them, and your relationship with them. At the same time, both your son and your daughter need you to be firm in this matter. They're going to be torn because of your ex-wife's actions, but seeing that you are not going to change your mind gives them real stability in an unstable situation.
For what its worth, I think you did very well. Anyone who has heard you speak of your children can tell how much you love them, and you already know to keep them involved with fun things outside of the cult world they are exposed to at their mother's home.
You're a good dad, AussieOz.
Tammy