if the PDC emulator in the root is synching to an outside source thats all you should need to do.
the PDC emulator in the child domains sync to the PDC emulator in the root
the domain controllers sync to the PDC emulator in their domain
the servers\workstations sync to the domain controller they authenticate to
check the w32time service is running on all computers.
You can check the time on a selection of machines to test using the script here