Thanks @acf-support
But I gave up!
No matter what I try or do, I just couldn’t get it to work.
But I have found another solution.
I removed the nested repeater and made it a normal not nested (stand-alone) repeater.
The (formerly) parent repeater now has an extra field with a unique number for every row.
The (formerly) child repeater also has an extra field for every row that corresponds with the unique number of one of the rows in the parent repeater.
All I have to do now is keep these numbers in sync, which is not that hard.
And now I can update the (formerly) child repeater with simple instructions like add_row or update_row.
This might not be the most ideal solution, but it works for me.
After trying and testing for a couple of days with nested repeaters, I got this solution working in less then an hour. And the results are almost the same.
