OP, I am 100% supportive of you but I do think you are going to have to detach, harden up and get a grip.
Your husband is an abuser.
Your husband has been abusing you and your children for a long time.
Your husband doesn't care a whit for your children and is continuing to abuse you all.
He wants to punish you and them for this move.
For years he could have stepped up.
That you would entertain his ridiculous texts is bizarre.
They are lies, manipulation and pure bullshit.
They bear absolutely no connection whatsoever to the reality of his fathering of his children.
I would be emailing the school the absolute truth, that he is weaponising the divorce to cause more pain and upset for the children and your eldest is very distressed. Give examples.
In your place I would source therapy asap.
I would also be very very calm.
Don't buy into this hysteria from your child, as it will only prolong it.
Keep with your narrative, daddy will be fine, mummy left because she was very sad as daddy was very nasty to her for a long time, and now both he and mummy will be happier.
That you love her and everything will be ok.
Then move on with the day.
She needs to learn to manage her emotions and your consistent reassurance and then getting on with the day.
Your children need protecting from him.
Encourage her to speak to a teacher if she likes.
Perhaps if the teacher hears what a horror her father has been for years, she might involve safe guarding.
Your daughters remark about not marrying a nasty boy and hurting her children is surely something you should be very proud of.
She HAS made the connection between being badly treated, leaving and that children are impacted by a nasty man behaving badly.
You are modelling strength to her.
She will get through this.
The very very worst thing you could do now is take on his manipulative bullshit and parent her though guilt.
This went on years and years, THAT should be your regret.
Your younger children are going to be fine.
Do not allow yourself to continue to be played by such an awful man who thinks NOTHING of emotionally abusing his old children.
Start texting him back how his behaviour is hurting the children.....every single time.
Nothing else.
You can do this.