Here's the problem - if you tell them and then you reconcile, they will never forget it. You might forgive him but they won't.
I would suggest you only tell them if A) he's moving out, B) they see something online and come to you, C) you are ready to divorce or D) you need the support of having someone in your family to talk to.
My ex cheated on me and I DB'd our way through that episode - I didn't tell my family because I KNEW my mom wouldn't ever forgive him. We had several more good years until he finally went off in a blaze of midlife crisis when he turned 50 - and no, my mom never forgave him that. She was madder even than I was. After all, he had wounded HER child and grandchildren and you know how mama bears are!
You don't have to lie if someone asks you (and I wouldn't). You don't have to ask your daughter to lie. But I wouldn't proactively share unless you feel you need their support.
