Confusion about Child Domains

Hello,

Very common setup: domain.com and a child.domain.com setup. My reasoning
for this is the following. Currently, if the VPN connection breaks between
the remote office and HQ the remote office loses certain network shares,
printers, etc. I was hoping by putting a DC in the remote branch I could
eliminate this. I thought they would be able to logon to the child domain
but validate against the parent and use a cached profile if the connection
is broken. Do I need to create usernames on the child domain though?

Thanks, sorry if the question is a little hard to understand. This
situation has been frustrating and there are too many thoughts in my head.

Jason