Hi Rhap,
I am the "other woman" in this kind of relationship & I believe it is so important to be honest with the children.
I'm my situation, my partner & I are always totally upfront & honest with his 2 children. His son was 5 when he first left his wife (before I met him) & his son didn't understand why Daddy had left, his mother told him it was coz daddy didn't love them anymore, which is the biggest BS I ever heard & she was angry at him for leaving them & thought it would be the best way to hurt him.
When we started having his children, we talked about this with him & after 5 years we still tell him that even though mum & dad don't love each other or live together, they will always be friends & they both love him very, very much!!
If you dont be honest with your children, it is them who end up being hurt or damaged by what has happened.
I hope I have helped.
Flinty