user heirarchy

I am trying to set up a new Drupal site with a complex set of permissions. It's a file management site that will allow a company (parent user) to create an account, then create accounts for their clients (child users). The parent and child users can then upload and share documents in a secure way. The parent user should have access to all nodes created by all of their child users, but nothing else. Child users should only have access to nodes they've created or nodes the parent has created for them.