OP you need to start documenting all this. Everything you can remember and add appropriate evidence which include, phone messages, emails, texts and witnesses. Download the Brightsky app and start journaling everything he's done including attempting to drink drive the children - I assume you refused to let him take them while drunk.
Dial 101 and tell the police what's going on. You need to start thinking - evidence evidence evidence - call everyone you can think of: Domestic Abuse services, Social Services, the police, the school and you need legal advice. Save all messages from him, I wouldn't block him if he's going to send abusive texts as you can show the police.
You need to report the threats pronto, that's why you need to contact the police. Get in contact with FLOWS who can give free legal advice and put you in contact with a solicitor.
Stop all contact between him and the children and send him a message:
I am tired of you neglecting my children and abusing me. From today I am ceasing all child contact. If you continue to harass and threaten me, I will contact the police. Please do not come to my home anymore, you are not welcome etc etc
Make it clear that you are stopping all child contact and that if he threatens you or comes to your home and causes a scene, you will contact the police. Do exactly that. If he turns up, phone 999. Keep calling 999. Keep contacting the police. Keep making reports. Keep documenting evidence.
Contact your local DV service for advice and support. Do a search like: 'DV help Kent' or wherever you live, take a look at your council website for details as they are usually detailed there.
Be strong OP. You have to protect your children from him.