You don't say in your letter how old your children are, and that could influence exactly how you handle this issue. Younger children, for example, would probably have a much harder time understanding why a parent would deceive them.
In any case, I urge you to avoid driving a wedge between your children and their father. Right now, you don't know for certain that he won't keep his promises (even if he probably won't), and making your children suspicious or putting them in a position where they have to choose sides could be very confusing to them. Divorce is hard enough on children; don't make it worse.
In other words, avoid letting whatever bitterness or anger you feel from your divorce spill over on your children. No, you can't change the past, but you can try to keep its damage at a minimum. The Bible says, "Peacemakers who sow in peace raise a harvest of righteousness" (James 3:18). If their father repeatedly breaks his promises to your children, then will be the time for you to help them understand and overcome their disappointment.
Above all, I urge you to do all you can to point your children to the only One who will never disappoint them and that is God, their Heavenly Father. Make Christ the center not only of your own life, but of your children's lives, as well.