This bit:
Other, and most important reason, FTCF has text my work phone 10 minutes ago.
"Actions have consequences. I'm so angry right now. My mum was out for a day with her friends and it was ruined because of you. You think you're always right but I wish you'd think about how your actions affect other people. My mother is upset which upsets me and that is having an effect on my DC. How dare you!"
This woman is deluded beyond all belief. Her mother has also very likely lied to her daughter about what ACTUALLY happened, but as others have said, there is an implied threat in her opening comment.
The rest of it though - "I wish you'd think about how your actions affect other people" - she just has no fucking clue, does she. No self-awareness at all. I do know that people like this exist, it just staggers me that they keep on believing their own version of life in the face of all evidence to the contrary.
I agree with almost everyone else - let the Police take this as far as they recommend, because of that implied threat.
I have a friend in a situation that started as a bit of chat in the school yard with another mum - my friend is a bit outspoken and spoke a little harshly about the other mum's oldest son, who is a feckless partner and father, and the other mum had a complete shit-fit about it. But, unlike normal people, she has escalated it to being a full-on war, involving the police, trying to get a restraining order on my friend, getting together with my friend's ex and his current GF to try and create a collaborative smear campaign - it's been 6 months of batshit levels of stupidity and trouble. It's coming to an end now, but only because batshit woman made the mistake of posting a pic on FB of not only my friend but 3 of the school children and another mum, and slating the head teacher, and saying that if she catches my friend so much as looking at her son (the one at school, not the oldest one) she's going to "take her down". Not even an implied threat, a full-on open threat. This is mad levels of escalation, but it shows that it can happen. The police are dealing with this one too.
So yes, let the police come, explain all the backdrop to this story, show them the text from FTCF and hopefully they'll put the wind up her and her mother - if it involves getting a restraining order on them both, then do it! You can't be having with this level of harassment. 
Very glad that you told your boss about the text too - it's ridiculous!
So sorry you've had to go through this. 