Since the affair was years ago and if you intend to work through things with your WW to continue your marriage it may be best to keep it to yourselves until the two of you can present it as a "past problem" instead of a current crisis.
That said... My daughters (in their 20's) were in the room when I got the phone call informing me of EX's infidelity. There was no way to shield them from it, so they know just about EVERYTHING that I know.
We did not tell my son (who was deployed overseas at the time) until it was evident the marriage was over. I'd hoped to be able to tell him (and the rest of our family) that we'd had a problem but worked through it.
All three kids are more bitter toward their father than I am, 4 years after the divorce.
Try to "tune in" to your daughter -- if she seems to be aware that something happened and she ASKS, perhaps honesty is the best policy.
All the best to you and your family.
Live in the NOW; when you realize you OWN it you have WON.