First, THIS ISN'T LEGAL ADVICE. No attorney-client relationship is established from this, etc. (Had to get that out of the way.)
I second Warren's recommendation to proceed with caution. Essentially, you can't use unpaid interns or externs for much commercially productive work. See the DOL's rules
on this. The DOL provided the most guidance to law firms in a recent letter
. That said, the letter can be interpreted to apply to your circumstances as well. Note: you don't want to be on the cutting edge, using interns in merely arguably permissible ways.