Why not add a second progress bar to your screen that would be used to display the child record progress? It would give the end-user a concise view of the overall process progress. Otherwise, you are going to have to do some trickery to get the actual number of records that will be processed. Here is some pseudo-code:
RecCount = ‘Select count(orders) from orders where order_date = today()’
RecCount += ‘Select count(details) from details where order_number in (select order_number from orders where order_date = today())’
ReCount then becomes the total number of records to process. You would increment RecordsProcessed with each order and detail processed.