To ensure that all child Opportunities contain the unique reference number from the Recurring Donation object and to prevent modifications after creation, follow these steps:

Create a Custom Text Field on the Opportunity Object:

Navigate to Setup.

In the Object Manager, select Opportunity.

Click on Fields & Relationships.

Create a new Text field (e.g., "Recurring Donation Reference Number").

Use NPSP Recurring Donation Custom Field Mappings:

Navigate to NPSP Settings.

Under "Donations", select "Recurring Donations".

Go to "Field Mappings".

Map the custom text field on the Recurring Donation object to the custom text field on the Opportunity object.

Ensure Field-Level Security:

Set the field-level security for the custom fields to ensure that they are only editable by the required profiles and read-only for others.

By following these steps, the unique reference number will be automatically populated and synchronized from the Recurring Donation object to all child Opportunities, and it will remain consistent for reconciliation purposes.