it can be locked only if child object is having master detail relationship. in the master detail relationship filed of child object to parent make sure the sharing settings is read/write selected Read/Write: Allows users with at least Read/Write access to the Master record to create, edit, or delete related Detail records
in the master detail relationship filed of child object to parent make sure the sharing settings is read/write selected
Read/Write: Allows users with at least Read/Write access to the Master record to create, edit, or delete related Detail records