<human>: Can you give me the gist of the text in a nutshell?

Brief background. I left my ex last Janurary because he was abusive to me (physically, mentally, and emotionally) We had be married for 4 years when I left. At that time Our daughters were 9mo and 3. 
It has been a rocky year and half, but I always gave him the chance to see the girls, supervised by me of course. He only chose to see them a handful of times, and even then he was high or drunk, or just wanted to focus on me and "getting me back". 

I did have a protective order against him for me and the girls but opted for only a 6 month on so he could calm down a bit and I could get the divorce started. He has never paid child support (even though he is ordered to by the state), Never called to talk to them, and hasn't even tried to do supervised visits like he is court ordered to do.

In this past year and half, I became very close/best friends with S (31m). He has been around my girls and was the "male firgure" in their life, Well S and I had decided to moved passed the friendship stage and are in a relationship, and he has taken on my children as his own. They call him Daddy. To my youngest He is the only daddy she knows. 

Now the problem. They are both young and they both call him daddy, since their "real Dad" isn't involved. My ex wants to call and talk to them because he "misses them". I have the OK from my attorney to let him talk to them under my supervision. How do I help my girls understand. I don't want to keep them away from their bio dad, especially if he gets his life together. I don't wanna be that women. But I know the girls have a very strong bond with S. I am so stuck, I don't want anyone to be hurt in this situation.
<bot>: How to deal with kids' (2 and 4) bio dad wanting to call after a year and half of no contact when they are already calling my best friend/boyfriend "daddy"?