5DMk2 for photographing kids ???

We currently have the oldy but goody original 5D. However, it is starting to show some wear and tear and it may soon be time to replace. We recently adopted a little girl with Down syndrome and so we have a new criteria to consider - photography a child who will with time be able to move very quickly. I was originally thinking to upgrade to the 5DMk2, however, I wonder if a faster focus and frame rate would be beneficial in capturing those elusive moments. The new 7D sounds intriguing, but I am a little leery of going back to a 1.6x camera. Although, it does appear that Canon has done a good job with the noise and IQ. I would appreciate your thoughts and experiences on using the 5DMk2 for kids.

• Peter