It is likely you do not have ownership of the child objects in order to apply permissions
On the security tab for the root folder, click advanced, then click the owner tab. Change owner to you and tick the box below that says 'replace owner on subcontainers and objects'.
Click ok to come all the way out of the properties for the root folder - go back in and you will now have the rights to change permissions on all the subfolders.
You can only do this if you are an administrator of the machine in question.