John-
This is something that could be done through customization. I'm assuming you are doing the invoicing through field services, you could put some kind of condition in the create invoices screen that if it is a job use the parent for invoicing therefore putting all the sub jobs on the invoice for the parent.
I will add this to our suggestion box for field services though we don't plan on making any changes to it anytime soon but when we do this will be considered!
-Michael