I find the 6D to have better focus than any crop camera (and I've owned them all). If the 6D can nail bighorn rams unpredictably smashing heads, it can keep up with your kids and track race cars.
Have you tried it on kids? It couldn't keep up with mine...
Like the 5DII before it, it does a decent job at tracking subjects moving across the frame, like the bighorns in your excellent image. Where the 5DII and 6D fail are when a subject is moving toward or away from the camera (away is worse). I just processed a burst sequence taken with my 1D X and 70-200/2.8L IS II of a gymnast running straight at me and vaulting from springboard onto the pad (which I was standing behind). All 26 shots in the burst are in crisp focus (lighting was pretty poor, shots were at 1/800 s, f/3.2, ISO 12800). The 7D would have gotten many of the shots in focus (but they'd have been unusable due to the ISO noise or the motion blur with a slower shutter speed. The noise from the 6D would have been acceptable, but after the first couple of frames, most of the shots would have been backfocused (and I'd have had far fewer shots, of course).
I found this to be true in diving and in long jump/triple jump. If I put zone AF on the 1Dx/70-200 f/2.8L II IS combo, it will track the diver from the jump, up in the air, and all the way down into the water. In long jump, I can get the entire sequence in sharp focus as the jumper springs off the runway and all the way into the sand, even at f/3.2. I am not aware of any other Canon camera that can do that, because I tried it with the 7D, 1D4, and 5D3, and none of them could do it as well. I sold my 7D, 1D4, and 5D3, and bought another 1Dx as a result.