Example of PPF with children?

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Hello All,

I'm working on a PPF that is essentially a fancy virtual folder with some right-click options, and for the life of me I can't figure out how to add sub-items to an object. I'm sure I'm just missing something obvious, but I'm out of ideas. Here's what I'm trying:

I know that this is possible. Many other providers do this, so I've got to be missing something.

Thanks!