Much as it must stick in your throat, I wouldn't mention the OW, IS.
I wouldn't even say at this stage that you've been hurt by their Dad, because at their age, their little minds believe that any hurt can be forgiven and they might think badly of you because you can't forgive a mistake that their Dad has made. Plus I imagine that when this fuckwit of a H gets around to seeing them, he will use them as a weapon to try to wheedle himself back - and you will look the bad guy.
Therefore, I think you should tell them that this is a joint decision and one that you both feel is best for them in the long run. You should ask your H to afford you the respect and dignity to back up this story in any interaction he has with the DCs - and of course he should not bring the OW anywhere near them for a good while yet.
As they get older, they will probably ask you some questions about the break-up - and at that stage (11plus I reckon) you can start to tell them the truth. It's at that age that they can learn that some things can never be forgiven.
I like the suggestion of reassuring them they are still a family and also at this stage of their lives, try to find some positives, such as it will be exciting for them to have sleepovers at Dad's new place when he gets it. Children tend to focus on such things and although that can be hurtful in a way, it is actually a good thing for them to see a silver lining.
This is going to be heartbreakingly hard for you I know - and you will need to see someone in RL as quickly as possible so that you can have a good howl at the injustice of it all, but throughout, have it uppermost in your mind that the lesson you are teaching your DCs will be a powerful one. That cheating and deceit are never acceptable and that you were brave and admirable once in forgiving the first affair, but that you would never want them to forgive repeated adultery.
You have done nothing wrong - and in time your DCs will see this. As adults, they will applaud you for your stance and they will have high expectations of their future relationships.