Active Directory Replication

On win2k Domain. i have root domain called CORDOMAIN.COM. and 2 child domains called
BRANCH1.CORDOMAIN.COM & BRANCH2.CORDOMAIN.COM each located at different sites. both these domains are connected through IP. apart from this i have 1 additional DC for Root Domain for redundancy. now the users at child domains are not same as what is there in root domain. according to AD replication all child domains will have the same users that are in root domain.each child doamins has a different bunch of users & groups. how do i configure DC so that if i add a user in root domain controller same users should get replicated to all child domains.

how do we configure replication ?