What legally needs to be done to have this set in stone that way if anything were to happen then everything will be exactly as the parents' wish?

My husband's lifelong friend and his wife are expecting a child and both of them have asked us to be the godparent of the child. They both come from rough backgrounds and don’t want the child to go to either of their family members as there are a lot of drugs and what not. My husband and I are the only people they really know that hold steady careers, a home, have healthy children and don’t do any type of drugs or any other recklessness that they are trying to keep the child from having.