There is a non script solution. I will call your Parent Record Projects and the 12 children �Monthly Budgets�
Make a table called Focus Project.
Make exactly 1 Record. It will be Record ID of 1.
Make a relationship where 1 Project has Many Focus Projects.
Edit the Focus project to choose a Proect and lookup all those common fields into it.
Make a new table with 12 records in it, called Focus Project Monthly Budgets. Link them to the Focus Project and lookup all the fields and create a formula to calculate the monthly fields.
Then create a saved table to table copy to copy the 12 monthly budgets into your !onthly Budgets Table.
Lastly, create a URL formula button on the Project to
1. Set the Focus Project.
2. Run the saved import with API_RunImport.
3. Refresh the Project Record