So for example, a folder with sub folders. Each folder may have ONE parent, but could have MANY children. I want all children of a parent to inherit the row level policies of the parent

You can go about this two ways, you can either have a

parent_id

that links to previous parents or you can use what they call a leaf node setup (this is more complex but with huge structures it's a lot faster). Do note that these are all just relational database practices.