The D800 will make better pictures than the D7000 under nearly all circumstances. That is a given. The D800 will do a bit better in low light than the D7000. That is a given.
But this statement really got me thinking...
>..I am almost certain that the 70-200 on my D7000 will not get the >desired results of my grandson running around on a soccer >field..
Now mind you I don't know how old your grandson is, and these images are not my best but some things I had handy from a few years ago..
So that is men's college soccer taken on a lazy afternoon Sept 11, 2011 and girls HS age soccer taken in Apr. 2011. I wouldn't submit these to any magazines, but they are probably ok for the web. I wish I had some other images .
The D7000 with proper glass is able to do the job. The only question is are YOU up to the job.