Try reading these :-
http://www.atomicobject.com/pages/Aggregation
http://en.wikipedia.org/wiki/Object_...on#Aggregation
http://msdn2.microsoft.com/en-us/library/395dc977.aspx
Theres no need to implement a collection interface unless you want to create your own collection class, or data structure, for simple aggregation this is not required, just use the languages array or collection contructs or classes.
If you mean parent and child in the literal real world sense then you probably want to model things using other OO constructs, for example a Parent and a Child are both Persons indicating inheritence. You then probably want containment by reference as some peoples parents may be other peoples children. Of course you should create inheritance heirachies based on functionality, so if there is none, in reality you would probably just model it with a single Person class.
http://en.wikipedia.org/wiki/Inherit...mputer_science)
Martin Fowlers
Analysis Patterns is an excellent introduction into such things. However patterns information is everywhere these days so buying the book is not necessary if you are prepared to dig a bit.
http://martinfowler.com/apsupp/accountability.pdf