A child association **IS** an association.
A child association is a uniquely named relationship between a single parent and single child node. The most common example of a child association in alfresco is cm:contains which is used to construct the tree of file and folders you see in Explorer/CIFS/FTP etc.
You can also have other sorts of associations with
a) different cardinality (many to one, one to many etc.) e.g. cm:reference
b) different constraints
c) primary and secondary associations.
