Broke NC

I hadn't contacted my exN for over 4 months except for emails about our divorce of which a copy goes to our lawyer(mediator).

My youngest daughter went to see him last Sunday and came back all ruffled. He had asked her to ask me for a note which showed what he still needed to pay for her until she was 21 (which is in just over a year) a couple of weeks ago.
I was livid as I knew this was total bullshit. How hard is it to figure out monthly payments x number of months to 21st birthday? I did not do anything. He kept asking her for the note and she was getting mad at me for not replying. I told her it was best for me not too and that she should stay out of it.
She came home Sunday saying she would write it herself because he had threatened not to cooperate with the settlement anymore and would hang me out to dry financially. Get his own lawyer etc...
My eldest at that point started criticizing me for the NC. Saying that is was hurting them as well.
I decided the next day to send him an email stating the amount he owed. Nothing more. I told myself that I was protecting my youngest from his bullshit but know that he will probably pull something else soon. Anything to make this settlement hard and tedious. I don't have the money for a lawyer. He will bleed me dry.

I don't know what to do. I can't tell my kids what he really is and why I need to stay away. They are young adults and that would absolutely leave me looking like the bad guy. Thinking now that I should forget about the settlement but that seems unfair to me and the children. I worked my ass off for (30)years and my youngest has 3 more years of university ahead of her. My car has about a year left and I'm scared I won't be able to keep a roof over our heads. But the hardest part is my kids not understanding why I want nothing to do with him.
I feel like I've done something really wrong in emailing him yet my kids feel better. This sucks!!!!!!!