Why do we go for parent child relationship
While establishing the relationship between attributes one can either look from business hierarchy point of view and the attribute higher in the hierarchy becomes parent of the attribute lower in the hierarchy. Parent and Child follow a one-to-many relationship. Example Time hierarchy Year > Month > Date. Here Year would be parent of Month and Date and Month parent of Date.
We can also identify Parent-Child relationship from database design point of view. Here in a table the Primary Key uniquely identifies the other columns in the table and hence qualifies as child of all the other attributes from the table, in the same ways as a child in real world identifies his father (at least the biological one).