Definitely go with the 7D. The 1D III is going to be way overkill unless you're a professional photographer and your son is in the big leagues. Take the huge difference in cost and buy a good lens (like the EF 70-200mm f/4L IS USM or the EF 100-400mm f/4.5-5.6L IS USM) and a monopod.
Also keep in mind that the 1D III won't take EF-S lenses (in case you have any for your 40D).


