Being seriously ill, taking care of a sick child, working from home and missing work to spend time with a client are legitimate excuses for missing work. Suffering through a serious illness at work decreases productivity, and if the illness is contagious, co-workers may get sick as well.
Kids often get sick, and someone needs to stay home to take care of them. Workplaces are generally sympathetic when it comes to sick family members. Working away from the office, as long as work is getting done, is a valid way to miss work. It is usually best to provide a simple, honest explanation for missing work as opposed to concocting a story and lying.