I assume you're concerned about whether or not the sub-contractors can include your client's work in their portfolios, right? I'll answer with that assumption in mind.
A typical contract includes an NDA for the client's project, a non-compete clause for the duration of the project (pertaining to competitors of the client), penalties for late delivery, and other usual stuff like that.
In terms of their portfolios, I don't mind that they include our clients' projects. I sometimes even encourage it. Personally, I have yet to find a client that has a problem with this arrangement. In my mind, this gives the designer more incentive to do good work on our behalf. Most of our clients understand that we may sub-contract out as well.
There has been one case where we've been the sub-contracted agency, however. The parent agency asked us not to display their client's project on our portfolio. We've honored that agreement with no problems.