I'm sorry but a few things about this story don't seem to make sense to me.
His children aren't "the past" as he puts it, they are the present and the future.
If he was serious about marrying you he would get a divorce, set a date to marry you, and encourage you to be part of his children's life as their step mother.
That would be my expectation. Also, his children should come first, they are innocent in this, not what you and he may want. If he said he wants to get back together for the sake of his children(even if he was drunk-the truth can come out then) encourage him do that, and step aside.
If you are still in love in 15 years after the youngest is 18 you can marry then.