I just want highlight two bugs that I've reported to bugzilla regarding the child account, activated following these instructions: Mailboxes: Sharing vs. Relationships » Zimbra :: Blog
These two bugs are quite serious, especially the second one could seriously compromise the privacy of the sender.
Bug 33636 – "No such message part exists" error when I send message with attachment using a child account
Bug 33637 – random attachment coming from the sender's inbox when I send a message using a child account
Please vote them if you are interested as well and post an answer here if you know other workarounds for these bugs.