kim that it is so very bloody sad. You poor thing. Firstly, try not to beat yourself up over losing it with the dcs. We have all done it and although it tears you up inside you have to put it to one side... and listen to your daughter telling you are a wonderful mummy. How lovely. I am sure she means it wholeheartedly.
Your exh is harassing you. Badly. And your little ones in fact too. Please keep all the texts and emails he has sent you and start keeping a log of all calls - times, what was said etc - and any visits to your house. None of this is acceptable.
You must show your solicitor/whoever is dealing with the contact agreement, the texts and emails.
As for leaving the dcs with some stranger at the fairground that is crass in the extreme and I would have been very angry and upset too. What a total idiot.
Until Aug 11th at least, can you try not to respond to any of his calls or texts. And if he calls at the house giving you verbal, don't open the door but tell him calmly that he is frightening you and the children and you will call the police if he doesn't leave.
You could send him a polite, non-confrontational email this morning (ie something in print) saying something along the lines of "I think it is for the best, for the children and for myself, to stop all physical contact with you until a formal agreement on access is agreed in early August. As you could see the children were very upset by what happened on xxxx day (when he drove off) and I do not want them to be put through such a scene again."
Keep it short, and maybe add, that if he wants to talk to the children he is welcome to telephone the house at 6pm (whatever) on a monday and thursday (say) and do so, but not at any other time.
Perhaps some other MNers will come along soon and advise you further on that last idea. It would only be to show any court that you have not tried to stop contact completely with the DCs.
He sounds very weak, angry and immature. And I feel sorry for him that he has roped his oldest daughter into giving you aggro too.
I am not sure what else to suggest. Just don't engage with him. Does he live quite close to you? Did any neighbours witness the children getting very upset when he drove off? If so perhaps you could get them to confirm that in writing. That really is an abominable thing to do to them.
Please look after yourself today. Write the email (and print up a copy to put with the others)
You could also try phoning Women's Aid on Monday to ask for some advice on how to deal with him.
Hug your children today and put on a sunny face for them.