While I am not familiar with your history with this man, I do have a few opinions about where you are in regard to the relationship with him.
1. You have a children together. You MUST maintain a civil relationship for their sake. Communication should be between you two, never "through" the children. Don't ever ask them to give him any message, and never, ever "grill" the children for information about him. Do NOT ever say anything bad to your child about him. Your children are half him and half you. That would be very hurtful and it is not your place to shape any opinions they might have of him.
2. He is an EX for a reason. Maybe for several reasons. Think about why you two split up. Sometimes women who are lonely settle for something and could do much better. You do not deserve to be used. Ever. He does not get to treat you bad in front of other people and be "nice" to you in private. He is manipulating you when he does this, and that is being used.
3. You will ALWAYS have some loving feelings for him. That does not necessarily mean you should be with him, much less marry this guy. You and he have a history, and that is hard to let go of. Go ahead and grieve the loss of this relationship and MOVE ON.
I don't know you, but I've been where you are. I wish someone had been blunt with me and told me these things. It would have prevented A LOT of heartache. You are in my prayers.