When a branch users and computers logs in, the RODC contacts a writeable DC for their authentication and caches their password provided Password Replication Policy (PRP) is configured. However, you can prepopulate the passwords cache on RODC with user and computer accounts before they try to log in at the branch site.
Extra domains make the network more complex to administer. In the past extra domains were created because of the limitations in creating a large number of objects in active directory and there was only one password policy per domain. Now active directory can scale to millions of objects and it has more than one password policy per domain. Before you create a child domain, ask the following question.
Why Do You Create a Child Domain?