Create new child records based on a date range

One method would be to have a table of say 48 records, numbered from 1 to 48. Those would represent the length of say a four year project. 

You can have an automation trigger to copy those 48 records in as a trial table to your project.  

Look up the start and end dates from the project down to the months table and create a formula that will calculate to the beginning of each month for each of the 48 months.  

So the first action of the automation could be to copy in those 48 records. 

The second action of the automation could be to delete any of those 48 records which are beyond the end date of the project. So for example if a project is three years you would copy in 48 records and then give it a haircut and delete off the last 12 records.