We installed nanny cams for our own peace of mind...
You can get them at Communica (or Electronica

- cant remember now but you can google both) in Midrand, and one of the guys there will come after hours to install, although they are fairly easy to install them yourselves.
You dont want them concealed. We told our nanny where they are in every room and that we dont want to see anything on the tapes that might upset us. That way they are aware of them from the beginning and (should) do things your way...
All the best, you are definitely making the right decision for your peace of mind!