Tim,
A couple of things are evident from this image.
Look at the shadow at the bottom of the image, right in the middle, at the bottom edge of the frame. This is the shadow of your lenshood, which means that you are shooting at the wide angle setting of your lens. 18mm.
From that it follows that you are shooting at a very close distance to the subject, and in that regard, the auto exposure electronics of your camera and flash are less effective: the flash will tend to overpower your subjects at close distances.
As per Glen's request, what
mode have you been shooting in? Does the flash head pop up by itself when shooting, or do you manually have to pup it up?
While Pa has suggested that you shoot in M
mode, this looks to me as if you already may well be shooting in M, but with settings that not correct.
Alternatively, there will be a setting for the flash that permits you to set the flash power to up to three stops over (or under) the correct exposure. I generally have this set on my cameras to about -2/3EV. If you have this value set to +1.5EV or more, then this sort of image would be an expected outcome. In your viewfinder, do you see an icon with a +/- sign?