Solost - I think in all fairness to your DC you need to tell them before Christmas. Otherwise they are quite likely to:
a) be disappointed when your H doesn't live up to his promise, because he has been told by OW that he has to spend some time with her on Christmas Day after all. This will either take the form of him being really late, or having to leave early.
b) ask him when he's coming back to live with you all properly as a family, which may precipitate a crisis on Christmas Day, something I'm sure you'd all like to avoid.
They probably have some kind of clue what is going on anyway - but even if they don't, I don't think that you all pretending things are fine until Christmas, and then telling them afterwards, is going to help.
Christmas is far enough away to get it over and done with now, allowing them to start to come to terms with it by then.
I don't know if your DC do Christmas letters, but you might see things like "I just want Daddy to come and live with us again" if you haven't explained the situation - which will doubtless upset you, at the very least.
I have always believed that children need to know the truth, even if it's just a simplified version appropriate to their age and level of understanding. Then they can process what is happening appropriately for them, not have to deal with one story, then another story, then finally get the truth and have to deal with not only that but the fact that their parents have been lying to them up until that point.
I am so glad you have your PILs on "your side" over this - if even your MIL is telling you to see a solicitor then it's time! Glad you're booking to see one soon.
Keep ignoring irrelevant texts - he's just looking for a sympathy reaction. Don't give him the satisfaction. And I hope to heaven that you don't sleep with him again!! That is over now - and anyway, EWWWWWW!! I wouldn't be wanting his thing in me after he's dipped it in her!!
Keep smiling kid, you're going great guns.