Upload & Sell: Off
Having had the opportunity to shoot kids for a good while now, I have not felt limited by the 5D2's autofocus or image quality. Although it can be said that the autofocus of the 5D3 is superior to the 5D2, there is definitely a price to pay for it, which in my mind is not really worth it for the type of shooting that I do (Still portrait, family, travel, etc). To me, the 5D3 offers little in improved image quality and higher ISO capabilities, which is why I have decided to hang on to my trusty 5D2 for a little while longer. YMMV however.
While kids do tend to move around alot and "erratically" as many may put it, if they aren't running towards the camera or really fast, alot of the AF shortcomings can be accounted and predicted for in advance. It takes some practice, but I have gotten used to it in the last few years that I have had my 5D2. For the price difference between the fair market value and the cost of a new 5D3, I would much rather put the money into more glass, which in my opinion is a much better buy depending on the type of shooting that you do. You can never go wrong with a new lens
The 5D2 is still a phenomenal camera, even after 4 years of it's introduction, and is still one of the best cameras Canon has ever come out with. Although many have moved on to the newer 5D3 and the latest and greatest, the 5D2 still provides great images that continue to shine on today.