OK.
This is abuse. This is harassment and threatening behaviour. You know it, HE knows it. Good that this twat dickhead man is stupid enough to actually put it in writing. Let's hope you can do something with it.
Can you please call Woman's Aid? There is a link at the top of this topic.
Defriend and block him from FB. No-one that threatens you like that gets to see your day to day activities.
The reply to this (IN YOUR HEAD!) is "... OR what? What IS this dick going to do?
He's gone from your life now, he doesn't get the right to rule you by tyranny. He never had that right, but just chose to take it anyway. Enough. No More.
Is the access to the children something you have agreed in court? would you be in breach of an order if you suspended contact?
Unless you can arrange for someone else to be there for the hand over of the DC, then I would suggest you cancel the access this weekend. You can't allow a bully like this access to you, under any circumstances, not in front of your DC, and tbh, I'd worry about their safety too if left with a man that sent messages like this. he has to be told this. it's unacceptable and inexcusable.
If informal access agreement, and no third party to do the handovers, text him with a message that says that due to unforeseen circumstances, the children will not be seeing him this weekend. Should he care to communicate with you with respect, then you will be happy to rearrange. All threats so far have been stored and backed up, and if there is a repeat of this behaviour, you will be taking them all to the police and seeking a non-molestation order to insist he stays away from you and your DC until he undergoes a perpetrator programme for his abusive behaviour.
If you do have an order in place, then please call WA and also talk to the CAB about your rights.
My instinct is to advise you to refuse him the space, time and opportunity to abuse you. No contact with you at all until he proves he can communicate with respect and manners. He calls you and starts ranting and raving, hang up. He calls back, you answer, he rants again, you hang up.
You always have the option of insisting all contact is via email or even solicitor. You can change your numbers and give instructions that he is not to contact you at work.
He wants to swing his big dick around, let him, but your's is bigger. 