You have the following options -
1. Go to the client deployment wizard and select "communication package"
2. PUSH DEPLOY this Sylink file to the clients from child company
3. All the clients will then communicate with the new SEPM server
OR
4. You can replicate or create the load balancer to the primary site, giving the priority to the main server
5. When ALL the clients from the child company communicates to the main SEPM (priority 1), decommision the priority 2 SEPM manager
Here's the article for Configuring a management server list for load balancing -
https://support.symantec.com/en_US/article.HOWTO81154.html#v8112658
Mark this as solution, if this worked for you.