Should I force my children to talk to their father?

I have two beautiful daughters ages 11 and 6. Their father and I were together for 10 years, never married. We broke up 3 years ago. Main reason for us breaking up was because of physical abuse and his alcoholism. My oldest daughters was witness to a lot of the physical violence.

I recently got married to and my children love my husband to death. They call him dad and my oldest wants him to adopt her. I try to keep the communication open with their father but he only calls them once every 2-3 months and when he does he is usually drunk. They both have expressed that they do not want to talk to him anymore. I feel that they are too young to make that decision, and that they should still respect their father so I have been making them call him at least once a week and talking to him for 10 minutes. Is this wrong of me? Should I respect their wishes and cut off ties to their dad?