Multi-Level Nested Subcategories in a DropDown

How can I list two levels of nested subcategories in a dropdown? I don’t want the parent to show.

So I have the following starting category structure:

-Property Type
—Land
—–1 Acre
—–2 Acre
—–3 Acre
—Houses
—–1 Bedroom
—–2 Bedroom
—–3 Bedroom

I would like a dropdown that will show:

-Land
—1 Acre
—2 Acre
—3 Acre
-Houses
—1 Bedroom
—2 Bedroom
—3 Bedroom

So it should show all subcategories of a parent in nested form without showing the parent.

Any help would be great!