shadow people and animals have been long thought of as spirits unable to fully manifest and those who can see them are gifted with what many people believe as a gift and sometimes curse called "The Sight" not the sight of the future but the sight of the world of the dead you may have this gift and may be seeing and feeling theese things because of that and possibly emathetic tendencies. Maybe the child died and felt lonley and lost and didn't know he or she was dead and then his or hers parents left leaving the child with a terrible sense of abandonment
Though it's generally considered unknown what shadow people are this has been my feeling on what they are for years now. And this explanation is also my interpretation of this particular situation.