I'm generally of the opinion that you shouldn't keep secrets from children like, they're adopted, or maybe one of their parents isn't actually their biological parent. That it's better to tell them often and early enough that it's something they're aware of, and not some giant shock when they're older and you tell them and/or they find out accidentally.
There's a large age gap between my two children, so at the height of the affair when my FWS had moved out, our oldest was aware of what was going on ("Daddy has a girlfriend, and you're not supposed to have any other girlfriends when you're married and have a wife."), and our youngest was a toddler and not even talking yet...
This was 6-7 years ago. FWS and I are legally separated, but we started living together again during the pandemic, and have been in a relationship ever since.
A couple of years back, when our youngest was probably about 6, it came up that a classmate's parent's weren't married, just "boyfriend and girlfriend." I mentioned that, technically FWS and I weren't married, but there's all different types of families. She responded, "If you're not married, then who is my mother?" I explained I'm still her mother, it just meant that her dad & I weren't married, but we still love each other and live together as a family. She seemed to undertand and didn't have any more questions.
A couple of days ago, we went to a baby shower, and I got dressed up and put a ring on my left ring finger. She noticed (I usually don't wear rings), and asked me if it was my wedding ring. I explained that it's my family ring, because it has three birthstones, for FWS & my two kids. She then asked why I don't wear my wedding ring. I explained I don't wear a lot of jewelry, but this was for a fancy party, so I got dressed up. She said that FWS always wears his wedding ring (it's true), and that I should wear mine, too. (I haven't since the affair.)
I just said that I like my family ring better because it's for everyone, but she just replied that I should really wear my wedding ring.
I'm wondering if/when I/we should ever mention what happened with FWS' affair, living separately between 2018-2020 etc.
She's only 8yo, so it feels really weird to bring it up now, but my worry is that her older brother or another family member might mention it one day, and she'll be totally caught off guard that this all happened when she was little, but too young to remember.
Also, a week or so ago, she randomly asked me if I knew any cheaters. I was caught off guard and asked what she meant, but when she explained, it was more like, cheating at cards, not having an affair, but it got me thinking...
I'd rather her not find out and have it be a shock when she's older, but randomly bringing this up also feels like a really inappropriate conversation to have with a kid that's still in elementary school.