You can't do it that way, I'm afraid. Alfred only supports a flat list of items, not a tree.
You can recursively call the same Script Filter via External Triggers, but it's up to you to manage the position in the tree and return the appropriate list of items for that level.
Here's a workflow I wrote that can drill-down into your Safari bookmark folders. But it works by first flattening the tree into a list and using folderID to select a sub-tree.
If you want to keep your data as a tree structure, you might try passing a "dotted path", e.g. 1, 1.1, 1.2, back to your Script Filter and then extracting the relevant section.
