Separated last year. Now sharing custody and living with dds aged nearly 13 and nearly 11.
I have a lover, very casual, he's a bit complicated. I've known him for a while, met online. The sex is amazing, lovely after years of married celibacy. I was on the phone to him last night, normal conversation and I have lots of friends so nothing untoward and it was a neutral conversation.
But dd1 (12) seemed a bit funny about it and asked who I was talking to.
This evening had to pick up the other one from something and got back to find dd1 looking teary and went up to her room.
She told me she had gone into my phone when I was out and read texts. There were some filthy ones from Mr X including one where he said I was a great fuck 
She was terrified about telling me that she'd read them. I normally delete but anyway I pw-protect everything and had no idea she'd seen and memorised my passcode.
I felt shaky, but stayed calm on the outside, asked her what she knew. I think she was mostly concerned I might bring a man into her life when ironically I would want to do nothing less. I reassured her about that. I also explained that I was a normal woman and that sex was nice; especially great with someone you love. But also could be just fun and was not dirty and nasty or anything I was ashamed of. I made clear he doesn't live near us. I told her how many times I've seen him (five) and that only one occasion was in our house when I was here alone.
I told her I didn't want her to lie for me but that I didn't want her sister or dad to know. I said that if anyone asked her she didn't need to lie but asked her not to volunteer information to dh or dd2 as it would be hurtful and difficult. Dh is difficult and angry; I really don't want him to know.
She was v apologetic about reading the texts and I made clear it wasn't on, without giving her a hard time. I said that I respected her privacy and wouldn't read hers (this is true and she knows it). But I said I wasn't angry with her, but with myself for not deleting the texts. I also said that I was glad she talked to me.
She is shocked, I think she thought she knew everything about me. She said she couldn't believe that there was 'a whole other person' in my life.
She was adamant that she doesn't think I've cheated on her dad because she accepts we are over. I told her I like my independence and only care about the three of us and our home.
She seems OK now. She is a bit teenager-y in the last year but we've always been able to talk about things. Has anyone been in this situation?
Out of my comfort zone and getting used to being a single mum.