How old are they? I think that’s the main issue. If they’re adults and you believe they have a distorted view of their father, I think it’s fair to tell them that. I also wouldn’t tell your MIL. At her age, I’d guess she’s highly unlikely to accept the information anyway. Whether the children do will depend on how you frame your response. It’s pretty sad if they are very negative towards their father. Perhaps you giving your honest opinion might allow them to at least consider a better relationship with him.