Oh, OP, I'm really sorry, and boy do I feel for you. My H and I are separated following the revelation of two affairs on his part. We spent hours discussing what and how much to tell our DCs (12 and 11), including some time with a family therapist. At first, I wanted to give them an age appropriate (very simplified) truth (e.g., Dad had a girlfriend and that's not OK in a marriage), as I thought the eldest would probably ask the question and I didn't want to lie. Speaking to the family therapist and my own individual therapist (highly recommend finding one for yourself), who also works with families in high conflict divorces, along with some friends whose parents had split when they were younger, I ultimately came to a different bottom line: (i) I would not lie to the children and (ii) It was important for my children's psychological health for them to have the best possible relationship they could with their father.
What this meant for us is that we told the children that we were not happy living together anymore (true) but that we loved them more than anything and would do what we could to make the transition as smooth as possible (we are staying in house, he has taken a flat literally 10 minutes walk across the river from us). When pressed for details, we have put up a screen of privacy by saying, "There are some things that are between me and Dad. I won't lie to you, but I may not answer all your questions. I hope you can try to understand that."
I won't pretend this is a perfect resolution. It really burns me sometimes, and our youngest clearly is struggling with the fact that the children didn't really see a lot of unhappiness and arguing before the split (our youngest actually thought we were joking at first when we told her - absolutely broke my heart), so seems to think I've just chucked him out for no reason when he's playing his "sad sausage" role. But mostly, I think it has been for the best for the children, and he has been a good father to them so far during this, frankly doing more with them than he had for the last year or so he was with the second, more serious OW, and telling me how unhappy he was without actually specifying that his unhappiness seem to be located conveniently when OW2 made herself available. At some point in the future, I will possibly answer the question differently if asked, but we'll see.
I wish you all the best. I'm so sorry you are dealing with this.
(And I'm really sorry, but do prepare yourself for an OW. The situation absolutely screams of it. : ( Believe me, I would never have believed it either until it happened to me.)