if you like, they can use the same login name, but i don't think this is a good option, because all people whould be able to delete the files from the others (since they are all the same user, an a user can delete his files!)
if every child has his own login, he could log in in the server using this, and keep his files secure (not that a child deletes some data from other users). with a simple script you can add keep the config of the samba-server up 2 date.
if you go to www.samba.org
, you'll find a lot of information about samba, with enxamples and howto's. tell me if i can help you, too