I would say it depends on the age of the children. Its important for us to remember that our children will grow up and understand much more in time to come. Ive found honesty with my child (age appropriately) has helped. The only person who knows the answer is your ex, maybe you can say you just dont honestly know.
At some point they will come to their own conclusions whether in 6 months or ten years.
The only thing I decided to stop doing was 'protecting' his dad against all the stupid things he did/does.
My relationship with my son is my priority, it should be my ex's too but if not he is the one who will answer in years to come.
My son is 9 and not stupid. He believes very little of what his dad tells him anymore. He loves his dad but 'dad' is no longer the deity he once was. Ex says this and that about me when drunk, I say little but my son knows what is what.
If my son asks me something I ask him back 'what do you think?' usually he comes out with the correct info and not what he has been told to believe.
I spent quite a while protecting his dad but then it struck me that my ex is an ass and if I didnt want my son to turn out to be an ass too I had to let my son see what is acceptable and what is not, by letting my son work out things for himself I am hoping to break any cycle that otherwise may have continued while my son believed his dad was always right and the best of the best.
We all do it in our own way. Remember, they will grow and understand.
I have decided that honesty is the only way to go. My kids ask I answer. Did dad cheat? I ask what do you think and then we go from there. I ask Why? I always try to be honest with them because I expect them to be honest with me. The Fleas(STBX) has cheated on me from the beginning of our marriage so I have trust issues. So in order to trust, I expect honest from them, so I give them as much as honesty as I can.
The others have good ideas. You know in your heart he cheated, but you don't know for sure, so I guess I would tell them you really don't know. My son would then push me into a corner and say "But what do you THINK?" I would tell him the only one that knows for sure is his father and that I can't speak for what he thinks or does. If he pushed it I would probably even suggest he visit with his father about it. I would know he wouldn't but it puts the responsibility of it right back there. I also have turned the question on my kids as Abbey and ladyrb suggested. Good luck.