It depends how old the kids are. You need to make sure, even if they are very young, that they understand that they had absolutely nothing to do with his leaving, directly or indirectly. Daddy loves them, he wants to be with them, but he needs to be healthy to make that possible. Mommy loves them, she is going to be there, take care of them, keep life going. It's okay to miss Daddy, because they love him. They can write him letters or make him pictures. I guess maybe though you don't want to make too big of a deal about it, make him into a hero or something. Get some movies and books, aimed at their age, dealing with being a family with a single parent. Sesame Street even has some Iphone Apps on the subject.
In a year and a half, who knows where you will be mentally and emotionally. right now, take care of the kids, keep reading and posting and learn how to live this new life. Each day we get stronger. When the time comes, you will know if letting him back in is right.