B., I've experienced the terror and hurt you are feeling, many years ago, and it turned out not as scary as my first fears.
Every big life lesson has taught me this: My attitude is the most important factor in how my experience goes. If my assumption is that the worst will happen, then my spirit will be knotted into a fist, and my experience will be one of conflict, hatred, anxiety, grief… all the things I don't want for myself or my loved ones. If I come to the experience with open hand and heart, much surprising good can come of it, and usually does.
That open heart can be hard to achieve, but it is so worth the effort. And what a model you will be offering to your children! Hang out with people you know who are most like this, and ask them for support. Prayer, meditation, good books, finding a spiritual community that functions on this basis, all can be helpful. There's also a wonderful system of communication that I have found helps me, and the process, immensely: http://www.earthlingcommunication.com/blog/the-nonviolent...
Kim D. give superb advice, based on first-hand experience. She sounds like an open-hearted person. Listen to her, and contact her if you need to for support and comfort. Her main points:
Talk to your ex first and be calm and attentive. You may be surprised to learn that his motives are actually well-intended, or at least based on love and longing for his children. And you might help dispel aggressive ideas he may have about having to fight to see his children. (That communication technique would be powerful here.)
It's incredibly important to put old hurts and angers behind you for all your kid's sake.
Think of your ex as one more person to love your children. It could be wonderful after you get past the scary parts.
A mediator is a great starting place. (This wonderful service was not available when I was divorcing, but I wish it was). You might be able to come up with a plan that will let your ex prove himself, and gradually increase the amount of access he has to his children.
Your children might benefit emotionally and even financially from having their bio dad as "real" family again. I would not assume this can't happen.
I wish you well. I hope you'll write again someday that things went much better than you thought they would.