My heart goes out to you and your family. I am in agreement with the previous response, that you must get everything in writing. Though everyone is in agreement right now, who is to say that with time, you and your ex won't be as compliant.
As for your 7 year old,explaining the situation in honest terms is good, but what she may need is action. Showing consistency, love, honesty, and open communication. Her life is going to be flipped upside down. Life is going to feel unstable. She may need it confirmed that you and her dad are still present, just in different ways now. She may act out, but just be patient.
Though I'm a little biased about counseling (since I am one), I truly believe that if you find the right counselor, it will be really benefical. And not just before you see problems resulting from the divorce. But be preventative.
THere are also a lot of books out there right now that may be helpful.
Sorry, i didn't mean to sound preachy. It sounds like you have a solid head, and a great love for your children. Remember to take care of yourself, you are going through a lot right now. Surround yourself with good friends and family. When you are taking care of yourself, that will make the enivronment more comfortable for your kids.
I hope God blesses you and your children during this troubled time.