File sharing on Lion

I have a iMac running 10.7 Lion. I am trying to setup the ability to share a couple external HD that are connected via firewire and USB. I can currently log into my MacBook Pro, connect to the iMac through finder and can see and access the external drives on the Macbook. I have both the iMac and Macbook Pro set up with seperate logins for each of my kids so they have access to their own files. When they are on the Macbook Pro, they can connect through finder to the iMac and access their files. My goal now is that since the internal drives are filling up with their photos, I need to move those files to the external HD.

When they do connect to the iMac from the Macbook pro, they can only see their home directory space. I would also like to have them access the external HD so they can start accessing those drives which have enough space. The problem is that they do not see the external drives. I can see them when I login, I'm the only one with administration rights. Is there a way to give access to those external drives when they are logged in? They are shared since I can see them when I am logged in.

Thanks in advance!