A child domain controller creates a subdomain in your active directory tree.  This is used as a security feature so a group of users can have separate policies and limited or controlled access to the parent domain resources. An additional domain controller adds a secondary domain controller to the domain.  It's recommended to have at least 2 domain controllers per domain to take advantage of the high availability features of Active Directory.

A child domain controller creates a subdomain in your active directory tree.  This is used as a security feature so a group of users can have separate policies and limited or controlled access to the parent domain resources. An additional domain controller adds a secondary domain controller to the domain.  It's recommended to have at least 2 domain controllers per domain to take advantage of the high availability features of Active Directory.