I don't have personal experience with this situation myself, but hoping the others who do could help with this question....
I am wondering if FA and her husband should meet separately from the children first to get their story straight (not a great phrase, sorry). The children will have lots of questions about what has happened, and FA is not going to be there to hear the answers if her H just takes the children. Doesn't he owe her an explanation first? And shouldn't she know/be part of the negotiation of what they are told?
He's enough of an arse to have just dropped out of the marriage with no warning, and I wonder what he will say to the children about why he has left. If FA doesn't know herself, how can she answer any followup questions when the children return from their visit with their father?
Huge apologies FA, I have no idea how you feel about this, or if you'd even want to meet him first. I only think he owes it to you, and that it would help the children if they got the same message from both of you. You have been fantastic in not slagging him off to the DCs, and have been way too hard on yourself in saying things that make this your fault (it isn't!!).
But when they start asking questions like 'are you getting divorced' to each of you (the DD11 and DD16 most likely to ask this), it would be better, wouldn't it, if the answers coming from both of you are coordinated. And at the moment, he's told you nothing really.
You don't deserve this, you really don't. For what he's dropped you in, you shouldn't have to be the sensible, practical one either--but like others have said, you are a mother, and you will do what you have to to make sure the DCs feel secure.