I feel the sooner children are told the truth about anything, the more accepting they are and less curious. You can give evasive answers and quickly change the subject when issues are “delicate”. My ex was absent a lot with my 3 kids, and now “my” 5 grandchildren only see him about twice a year. I have not badmouthed him, just told the truth when asked, and they all see him already, for exactly what he is. He admits that he was a crap father ( his words) yet still makes no effort to be a better grandfather, so he is reaping what he has sown. It’s a learning curve for kids, that life is not always perfect and people will let you down at times.