Hi OP I've only just read your story today and I'm so angry on your behalf. I've seen your STBEH many times before, but his bloody family???
It reads as if you are communicating with him quite frequently, my advice would be to pull that right back and have as little as possible to do with him. Don't tell him anything he doesn't need to know, even things like how the kids are etc unless it's an emergency.
If he takes the kids out for a few hours, make sure you are out for the last hour and if he still has a key expect that he'll return early and use it, so everything private locked away, nice tidy house, flowers etc (I'm not falling apart without you) if you can legally change the locks, do it.
I understand he was your best friend but he isn't now, he is currently your worst enemy. In the future you might find some common ground but for now don't be so open with him, so find someone else to deliver the calpol etc. Yes, I know it's hard.
You mention 'she says xxx' etc, stop talking to him about her, stop asking questions about her. He wants to talk about her, good or bad, and as his 'best friend' you are the one he wants listening. Stop listening. I know it's hard because it's instinctive to want to know about her-why her and not me- but it feeds his ego when you listen.
Likewise, stop spending time with him and the kids. He probably can't 'dad' on his own without you and needs you to be his wing woman. Fuck him! He made the decision to leave and single parent hood is one of the consequences. Your kids will survive without you for that short period even if they only get toast and marmite. Your daughter sounds capable of sticking up for herself and her brother.
Get therapy. Get therapy. Get therapy. It helps and you deserve it.
You have done an amazing job at one of the most stressful times of the year, hats off to you, you're an amazing mum. Wishing you and the kids all the very best for the future x