I was married to someone when I was 23 for 5 years. We had no kids and it was an amicable split. Benn married now to dh for almost 7 years and we have 2 dds together, the eldest being 5. I haven't really thought much about telling dd1 about being married before: I think she's far too young to know about it now as she's only just getting to grips with how relationships work and she obviously has a very simplistic view of love and marriage.
But I was talking to my BIL yesterday - he was married before he met his 2nd wife and had no kids with wife no.1. His eldest ds is 11 and will be going to a school in sept where BIL's 1st wife actually works. We were wondering whether BIL should tell his ds who she is and what happened before something comes out by accident. And that made me think what I would tell my dds and when I would tell them.