9.04.506 - User Defined Tables (Parent/Child)

I am trying to build a data entry screen that has parent and child tables. The "header" information is figured out however the child info have no fields that would I can consider unique. What I would like to do is create a counter field (similar to OrderLineNum) for my child table. I am unsure how to get that to occur in the child key field 1.

Any thoughts

Thanks,
Rick Troutman
Ridge Engineering