I have two children who have some personal devices (mobile phone, tablet) associated with their profiles, but I also want to assign shared devices (family iMac). The parental controls appear only to allow me to assign a device to one profile or another. Once assigned it no longer appears in the list of assignable devices for any other profile. 

A related question I have is, what is being timed? Is it the profile's access to the internet via a device, or the device's internet access time regardless of the profile to which it is assigned?