Preventing Child to Child replication

hi all,

we have a Single Forest consisting of about 30 Child Domains.

the Design is to Service the Exchange and Lync for all Child Domain Users

we have 30 Separate Sites too

but in relation to Routing of Network and Firewall Designs Child Sites Do Not have Access to each other so there is no Route from Child 1 to other Childs.other Childs are same.

In AD Replication i see Child Contexts cannot be synced to other Childs.is there any way to preventing child to child replication ? can i set to only replicate from Root Domain ?

Can you Guys please help me on this ?

Kind Regards