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...I was thinking having a nice, simple setup with 45 degree angles toward the family from either side of the camera to avoid any shadows.
Actually, that might give you crossed shadows on the background, rather than no shadows, which is usually not a good look. "There is only one sun" is an oft used rule of thumb for positioning lights, as a look to try to emulate. Properly placed shadows give subjects "depth" and "character."
If you're only doing this kind of thing a few times a year, I think your two flash guns are all you need. If you live near a shopping mall that has a "Photos with Santa" area, take a look at how they're doing things: often one light centered over the camera in a small pan reflector, and one other light to one side for a bit of subject definition. The latter may be in an umbrella or soft box, but it may also just be in a standard reflector.
You can come close to that with your flash guns by putting one over (or close to over) the camera position at a height that will give a small shadow under the nose of a subject looking straight at the camera (but not so high as to cast a strong shadow in the eye sockets), and the other flash in an umbrella at camera level and far enough to the side to give a little shading on one side of a subject's face.
If you're shooting in a typical home-sized space, you could also turn the fill light up and to the rear to bounce off the back wall and ceiling for some really soft and even fill.