Your children are going to have this guy for a father for probably a long time to come. If he's the jerk he sounds like, then you have two possible strategies: one is to limit his access to them (the post above gives a suggestion in that vein.) The second option is to help them develop skills in managing the fact that they have a jerk for a father. I recommend you do both.

Since your oldest is becoming "an emotional wreck" when her father gets through to her, she is sorely in need of developing those skills I mention in option # 2. He's not the only jerk she's going to have to deal with in this world. So learning the skills will make her a better, stronger person. I'm not expecting her to become made of steel, but it is not necessary for her to keep becoming a wreck. Don't make her sanity depend on her father being kept away from her. That makes her powerless. It's great that you want to protect her . . . but help her learn to protect herself.

You have a parental responsibility to shield her from adult B.S. when that's possible. But it's not always going to be possible. He's resourceful, and he'll find ways to get through to her. Teach her self-defense. Show her that she has options . . . that she can form and defend boundaries. Don't dictate what she should do, but explain that she does not have to be a sitting target for her dad to take aim at. If you don't know what that means, then get her some competent counseling.

Seeing her upset validates your position that this guy needs to be gone from your domestic life. But be careful you don't take too much satisfaction in that. Be careful you aren't a little too curious to read those poison texts that your ex sends. Your daughter can block him from texting her. Don't tell her to do that, but tell her it's an option for her to consider. Don't let her get addicted to drama. When she's all upset, don't console her excessively. Give her practical ideas about how she can limit her father tormenting her. Encourage her to not look too long at poison texts. And don't you look at them either. That removes some of the energy that fuels her being upset. Change the subject. Explain to her that decent parents don't put a daughter in the middle of their failure at marriage. Then find something for the two of you to do that's positive.

Cesar Milan (the dog whispered) says that, if you pet and caress and cuddle a dog when it's shivering in fear, then you teach it to be fearful. You reinforce the fear. I believe his philosophy can be applied to child rearing. By all means, validate that her father is wrong to send her upsetting texts . . . that real men don't do that. Then explain to her that, when people act very wrongly, they totally deserve to be ignored. Help her see that she has options . . . that getting upset is not her only option.