Form to select multiple items

[Bparham]

I have a parent table ( seel below ) Any time we place an order with a vendor, that information gets put here. I am trying to track when the package comes in, who checks it in and so on. I have set up a child table as a form to do this. When I select the PO number, I have other fields fill in based off that number. The area I am having trouble with is the PARTS. Each PO could have 20+ parts and I would like to be able to select which part came in and how many of that part came in as well. HOW DO I DO THIS?  

I would like the end result to look like the highlighted. Without having to check in each individual item.