Hi again Hannah!
I'm guessing the faces come out dark for two reasons. First, the high contrast of many outside scene means the camera will try to averag the total exposure, and the faces will get darker. Second, often facial features create shadows in the faces.
You might give us a link to some examples so we could confirm what the "problem" seems to be.
A Solution would be to use your camera's flash. Even though the scene is very bright, using the flash in Fill mode will help to make sure the faces are propely lit and exposed.
You can look here for more information:
http://www.digicamhelp.com/taking-photos/lighting-techniques/fill-flash.php
