I'm so sorry...I can't imagine how much this hurts. Even though you know it's better you found this out now, I know it doesn't make it feel any better. I have a friend was involved with her ex on again/off again for years--she became close to his children he had during his 2nd marriage and continued to have contact with them (even now) for the kids' sake. It's okay to still have contact with them if it is for their well-being also. I'm sure they are hurting, too, and need a chance at some closure to their relationship with you. I would talk to their mother about this if she is receptive to the idea. Your contact with the kids whether face to face or by mail does not have to involve their father.