This is what I love about this forum. My initial reaction was to keep it non-emotional and factual. Then after reading through these posts from people more experienced (even a lawyers view!) what is best for YOU is more clear.
Write and rewrite and rewrite. But don't send. Get to a point where you have the facts in writing for the court to review. Run it by your attorney. Take a screen shot of that rental ad and keep it as evidence. Keep the party picture. Keep every bill for the kids. Have all of this and be prepared to present it at court.
Fact is: your Ex already knows the facts. You're not telling him anything new in this email, but as others have said, you may be tipping your hand. Don't give him the opportunity to do damage control ahead of time.