Father playing games

My ex has decided to not be in my kids lives. Only when he feels like it. After not seeing them for 6 months my daughter told me she is going to have dinner with him. I assumed it was all three of the kids but it turns out it was just her invited. My kids are 16, 14 and 12. My 14 and 12 year old have had it so rough, my 14 year old has been severely depressed and I can't get him to school. I keep thinking I'm going to find him dead in his room. My 12 year old has been a bit lost too and the last time he saw his Dad his gf was making fun of the way he talks. Not in a joking way but a "why can't you talk properly" way. He came home in tears trying to tell me what had happened, he was sobbing so much because his dad didn't do anything. I personally wouldn't mind it if their Dad completely disappeared because it would be easier for my kids to deal with knowing that he doesn't see them because he's not here rather than think they're not good enough or that they've done something wrong. Now he's playing these head games with his own kids. I don't know how he can live with himself I wouldn't be able to do that. I feel so angry yet heartbroken for my kids at the same time.