Most Helpful Guy
The legal way to do it is to take him to court. Have the court put in the order that he picks the kids up at a disclosed location. Also have that the girlfriend can't be with him when he picks up the kids. I've seen it done. I work at a law firm.
As a father that has to pick up his son and drop him off at his moms house I deal with this a lot. Her new husband gives me dirty looks and tries to intimidate me. I used to let him get away with it. Until one day me and my son had a bad day. Nothing we tried to do worked out for us or they were closed. I was in bad mood and the new husband tried some crap with me I stood nose to nose with him and gave him a piece of mind. He stepped back and wanted to swing at me. I told go ahead. I would have kicked his ass and had him put in jail. Then I would of had the leverage I needed to get primary custody of my son. My ex knew what I trying to do, so she stepped in and stopped him. Now he doesn't come to the door when I pickup/drop off my son.