Depends on many factors. When a marriage doesn't work, staying together for the kids might backfire. Constant fighting, resentment and modeling a bad marriage for the kids, hurts them (specially the fighting). My neighbours were like that. He always was shouting, hit the walls and doors, was aggresive. She remained married for their child. Well, the child ended becoming anorexic and needing therapy. Divorce would have been kinder on that girl. the constant fighting was stressful even for my parent who just heard them fight. I can't even imagine how bad it was for the girl.

That doesn't mean that all marriages are like that. A good marriage is good for children. But a bad one is not.

And for the record, I'd like to think that most marriages would work. Or I least I would like to hope they work. But when the marriage is not ok and therapy for the couple doesn't fix it, divorce might be the best option for the family.