The height discrepancy between my boys isn't quite as great, but I took a lot of pictures where my older son (who, luckily, is freakishly flexible) stood in what he called a "horse stance" (a martial arts term for having the legs very wide) and I shot them from the waist up. I have extremely short daughters who were 11 years his junior and this was what worked.
Something you might try to get them in the mood is to let them do a few silly things first, such as this:
no end to the silliness on Flickr - Photo Sharing!. I also often take a "silly face" picture or two--especially when I'm working with all four of mine--before I try to take the real pictures. It helps relieve the weirdness of being in front of the camera.
Definitely look for either overcast skies or open shade (side of a building, for example), where they are shaded by still get light in their eyes. Think of having them stand inside the door of a barn or garage so they face toward the light, but don't have to squint to understand what I mean by "open shade."
If it's a little too shady or cloudy, you can try using your pop-up flash set to -2 for just a touch of light in their faces w/o that flashy look.
It took a while, but my boys did catch on to the fact that if they just cooperated, the photoshoot ended much sooner.