If you want to perform IDoc inbound processing in the background, you require a background user in every child system that you can use instead of your own user name to schedule the background jobs in transaction SM37.
In the child system, choose Tools → Administration → User Maintenance → User (transaction SU01).
Create a background user in accordance with the naming convention for system users (such as CUA_PRD_BACK).
Assign the user type System to the user, and deactivate the password (see Logon Data Tab Page).
Assign the role Z_SAP_BC_USR_CUA_CLIENT_BATCH to the user.