Lookup from another Child Document on same parent

I’m working on a customization to “Customer” and am curious to find-out how I would implement a link between Child Documents.

In the customization so far, I have the following item(s):


What I would like to do with the screen above is to fill-out the “Merchant Accounts” child document, then use those entries as a lookup on the “Terminals” child document.

Bonus points if someone can describe how to, instead of two child documents, I can use a tree view or similar to create a series of merchants, then off of each merchant (capturing several pieces of information) create one or more terminals, if it is indeed possible.