I would have given a reason from the start that was more like:
‘I felt he treated me disrespectfully’
or ‘I felt he wasn’t honest’
or ‘he didn’t keep a promise he made me about something that was important to me’
and then I’d think that the facts around that wound naturally come out when they were ready to know them.
i believe in that maxim that children only ask what they are ready to know, so as long as you listen carefully to the exact question they ask and answer that, you’ll be ok.
maybe move now to one of those formulations?
however, have you worked through your feelings about it?
it’s important you keep to the facts and don’t get drawn into telling them a lot about how awful it was for you, even though I’m sure it was.
my mother did and still does this a lot around her marriage and it’s end. I have boundaries now, but the reality that she used me as her therapist and emotional support from 11 or so onwards has had a permanent negative impact on our relationship.