Neither of you should be discussing the "reasons" of this with your kids... period.
It is an adult situation that kids simply can not comprehend. http://www.uptoparents.org will help explain. I suggest you forward the link to your H as it will put you both on the same page.
Keep the kids out of it. Answer their questions, NEVER degrade their father and he NEEDS to do the same. This isn't a contest, you both are responsible for raising them, put the A / OW issue to rest when the kids are present. Simply love them and support them as their parents, it is what they truely crave.
