They have to be cared for or they will die. For example, in an alcoholic home, little Sally has to worry about whether she can bring friends home - because daddy may be in a bad mood and embarrass her. Such events are training her in codependent thinking, the art of anticipating the other person. If mom is physically ill, Teddy has to worry about exerting her. If daddy is angry and controlling, Timmy needs to worry about pleasing him to avoid punishment and humiliation - and to get his conditional love and approval.
Children are naturally egocentric.
That means that they see the world revolving around them. If mom and dad fight, children feel that it is somehow their fault. As in school in an attempt to keep the parental marriage together. Another child may have an abusive, or simply overactive older sibling. Or, perhaps daddy is depressed.