Go ahead and tell them - but do it in a way that doesn't destroy the image of their father. Tell them that Daddy has decided to make his life with someone else - it's hard to explain why - it's just the way it is, but he is still Daddy and he still loves you children the same as before and he always will. You don't have to specify when Daddy met this other life partner but if the kids ask there is no reason not to tell them that he has known her for a long time or whatever. Now the kids are going to be mad at him for leaving them but at least they won't be mad at both of you. This is what I told my kids about my ex. They were pretty young at the time so I don't know what they understood. But by the time they were teenagers they knew all the facts from other people anyways and they knew that I never hid anything from them. I think if I had covered stuff up they would have experienced that as another betrayal of trust when the information came out later (which it always will). My kids definitely lost respect for my ex but not because of how I bad-mouthed him (because I didn't) but because of his own choice to abandon his family. There's not much you can do to make that look good!

I agree that hiding the affair from the kids can backfire. There's always some relative in the wings who will like the real story and then your kids can really resent YOU for hiding the facts from them. They feel they have a right to know everything because its THEIR lives that are so hugely impacted. While I don't think you have to give them details, I think you do need to give them an accurate rough outline of what actually transpired.

I think you should wait until they're old enough. If they are going to be looked after by your husband's new partner, you don't want them hating her guts any more than they would naturally do. My mom told us kids about Dad's affair and then we had to go to this family-ripper's house every week. I can't tell you how much we hated that woman for breaking up our home. She never had a chance with us. I don't know how much different it would have been if we didn't know about this till we were grown up. But I suspect we would have been a lot happier.