I agree with William, it's not so much about the gear, but how it's used. The good old 50mm f/1.8 II is still my go to when I first start out a session []


The type of camera body choice probably wouldn't be much problem, since the photographer probably would rather prefer to use their own that they are accustomed to (unless it's a huge upgrade). But if you needed to choose one, 7D, 60D, T3i, T2i, I wouldn't mind working with any one of those over the other. I'd miss the better AF performance from the 7D, but they all use relatively the same sensor so no drastic change there in quality. You just won't have the bells and whistles, which you don't seem to be asking for.Lens choice would have more impact.Better just to get the essentials if on a tight budget. T2i?


The Tamron 17-50mm f/2.8 is a decent all-rounder. From your past experience on working with these kids as their photographer was the focal length range ever a problem? If you felt you wanted more reach, the 85mm f/1.8 would be a nice extension, plus a good low light performance. But if it wasn't ever a problem the Sigma 35mm f/1.4 might be a better choice. The focal length would be just smack in the middle of the range preferred for your setting, so less burden for the photographer to move around to compose, plus the better low light performance than the f/2.8 on the Tamron.


But from the sounds of it, you're struggling more with actually keeping up with the kids as they move around. The Tamron 17-50mm f/2.8 focusing speed honestly to me is horrid with fast moving subjects in low light settings. You can have the widest aperture with the highest shutter speeds, but it won't do any good if the focus is trailing behind. I use it with my 7D, so a better body isn't going to solve that problem. It's the lens design. In that sense I would get the 85mm f/1.8 with it's fast and accurate focusing. You might be working outside a comfortable focal length, and more burden for the photographer to compose, but at least you'll capture the kids experiences in focus []


So I would work with T2i + 85mm f/1.8 + 17-50mm f/2.8 combination. 85mm f/1.8 for fast motion shots, and 17-50mm f/2.8 for relatively slow occasions where you want to capture a larger wider scene.


Just my 2cents!