OP I’m so glad you’ve acted! As I was reading through I was getting more and more worried for you - it sounded like you were going to make excuses for him and end up staying because you were scared.
I appreciate it’s going to feel easier now to cave to his demands, because it’s scary having a blood harassing you and turning up. He will try to guilt you, lovebomb you and potentially threaten you and your kids when he realises you are serious.
Stay strong, you have done the right thing. Practical steps now:
Log a call with 111 as he’s already shown signs of stalking and harassment
You have arranged to meet another mum to collect the kids - can you have someone come home with you, just in case he preempts this and decides to wait at your house rather than go to the school?
Let your neighbours know you have broken to with your boyfriend (send them a photo of him) and that he’s now harassing you, so if they see him turn up please call the police. If they ask them if you’re home, ask them to say your choice of they don’t know/you moved out/you’ve gone away on holiday and don’t know when you’re back
Tell your kids if they see him they are not to speak to him
Let friends/family know what is going on. If you can all stay with a friend or one can stay with you that would be a good idea
if there’s even the tiniest of remote chances he could have gotten a key cut for your place, change the locks.
Use a security chain at all times, or put something in front of the door.
If he turns up again, call the police. Don’t engage with him beyond, if you want to, telling him ONCE to leave. After you’ve told him once, don’t give him further chances. You made it clear on your message, so you absolutely do not need to even give him one chance before you call the police.
Let any calls go to voicemail (consider unblocking him if you want to gather evidence). If you unblock him, do not reply, do not read them, etc. ask a friend to review and say if you should show them to the police if necessary.
Yours and your children’s safety is paramount.
Once this settles down, it may be worth looking at the freedom program mentioned by someone else, as you have ignored a lot of red flags and have felt too scared to end things. Hopefully this can help stop you from being in this position again.
You’ve got this - keep posting if you want anonymous support. We will all be here for you.