I own a 70D & was shooting with a 5D2 last week.
The 70D is way better at the whole focus tracking/speed thing [mine is out but that is another thread]. The 5D2 focus system is not the best, hard to grab the right spot for you composition some of the time & it's really slow in comparison to the 70D but the images are very nice from the FF sensor.
If you shoot your kids indoors, in low light or at high ISO the cameras are about the same [ok, the 5D2 is better but not by much, imo] but you'll fit your kids in the frame on the 5D2 easier. Buy a wide angle lens like amfoto1 says and save the full frame want for the next upgrade
Also I've never had as SD card fail, had CF card go down mid shoot...





