Really depends on the "why" - as has been said, if it was simply (HA, as if anything is simple!) because of a loss of attraction or the gloss had gone, i would put my all into trying to save it - both myself, and through counselling etc. But ultimately, happiness has to win out - kids know a lot more than we give them credit for

if it was an unhealthy relationship (abusive - physically or emotionally), i'd be gone. i grew up as a child in that sort of situation - mum was always scared and crying, we were petrified to speak half the time, i used to BEG my mum to leave so that we could all be happy. I'd not put a child through that

as to the attachment thing - if there is a situation where you can't live together, but your children are very attached to the other parent, why can't you arrange a generous shared care arrangement. your children will ultimately adapt and be happier if their parents are happier...