there is a reason why gymnastics, swimming and track are listed by Charlie and Tudor as the best development sports out there.
Gymnastics for strength and flexibility ( perhaps not to the extreme as the kids at the highest level have)
Swimming for endurance ( not to say your kid should be doing the kind of overtraining most swim programs for youngsters advocate and result in life long shoulder issues
Track for speed and strength. Again speed means under 100 meters and much less younger a child is / does not mean distance training for cross country or stupid crap I see most Phys-Ed coaches give " lets begin practice with a 5 k run"
Sounds like a good book Flash. You are always researching something interesting.