We all know that for a kid having just a parent can become quite a problem. The ideal situation is when both parents can take care of the kid and they all life happily as a family.
Still ... there are situations when living in a full family might not be the best idea. I would think about one of the parents having blatant affairs outside the marriage, alcohol and drugs abuse, maybe fights in the family (an abusive husband).
Do you also consider in this case that it's better for a kid to remain with one parent? Aren't they serious enough to cause such a breakup? Do you have any experience in this?