If we have custody of a 2 non biological children, but the paternal side want to fight for the custody, can we file for adoption?

We have had 2 non biological children for over 1 year now. The mother refused to do anything after DHR said no contact. Both children under 12 years old but not much want to stay where they are but the paternal grandmother keeps trying to get the biological father to fight for custody of them for her but she or any other family wants them. Father and biological grandmother have been through all DHR meeting and never took custody of them but I've already in 2016 been to court 3 times with the father. Career student who has porn on his computer and when the children visit they tell them that they are going to spank them or have me spank them. What I do at my house is my business. They are almost 10 years old and he only visits when it's convenient for him. Every time they visit with their father the behaviors get worse, cry and beg not have to go with the whole paternal side of the family.