I have a friend, who told me this clearly:
If my girlfriend / wife ever betrays me and you know it, I do not want to know.
I respect that because he is my best friend.
As far as my experience, most people do want to know what kind of person they are living with.
Usually is the right thing to do.
Now, if hypothetically, my friend's wife was having an affair with my husband (I have a wife this is just as an example) it touches me, I would not care any longer of his request because is a devastation that touches me personally, then secondary him. So I would tell it all.
See it like this: how much more are you willing to sacrifice your peace and mental health on the altar of pain?
This is something that weighted on your soul for how long? 15 years?
Do you want to carry it until the grave or to bring you prematurely to it?
If you answer yes to those questions, then do nothing, that is also a choice. I think you were the victim here, so is the AP's BP.
You are a BS not the AP's friend holding his/her alibi towards his/her infidelity to their BS.
What can you be afraid the AP can ever do to you more? He/She already destroyed you life, identity, Love, past, present..... do not let him/her destroy the future too.
You are afraid the kids will find out?
Lies and betrayal will always come out in time, no matter how the cheater thinks they are gone, it's a curse for life. It will come out.
Are you afraid they will judge you? What about? You proved your love for your WS was strong enough that survived the ultimate violation a spouse can inflict to your love and bond: betrayal.
Tops, they can be disgusted with your WS, and proud of you. They are your children so in time they will forgive the WS, it's natural, but they might never forget the secrecy once they become adult.
They will understand but they too probably want honesty from their parents at some point.