English law mandated that all adult residents of the English State be members of the Church of England(Anglican). Virginia was an English Colony from its founding until the revolution. English Law prevailed there.  All adult residents of the colony were members of the church by law. Each parish vestry(12 elected members) operated as a defacto local governments, even as courts. The Virginia House of Burgesses and the General Assembly were formed in 1619 and began to legislate and set up courts and other institutions, but they were dominated by the the King, the Kings appointed governor, the governors council and to a great extent legislated in favor of the King, in conformity with the laws of England and the Church. 
To answer your question directly the Church appropriated the power to grant guardianship of orphaned children to itself. 

> Date: Sun, 14 Jul 2013 14:29:19 -0400
> From: [log in to unmask]
> Subject: [VA-ROOTS] Researching Virginia Orphan Books 1700 - 1735
> To: [log in to unmask]
> 
> I am researching microfilms of Virginia Orphan Books, can anyone tell me why the parish church wardens would grant guardianship to a relative of a young child.  How do the parish church wardens get permission to grant guardianship of a child.
> 
> 
> To subscribe, change options, or unsubscribe, please see the instructions at
> http://listlva.lib.va.us/archives/va-roots.html
 		 	   		  
To subscribe, change options, or unsubscribe, please see the instructions at
http://listlva.lib.va.us/archives/va-roots.html