With any shoot involving a baby or toddler, the photographer is completely at the whim of the child. It the child who is really directing the shoot. The little guy we had was an Energizer Bunny of energy moving around the set -- lot's of energy, lots of good expressions, but darting about so fast that my camera could not keep up with him, and when it did, the lens often did not achieve focus. To top it off, I was working the aperture around f/2 to keep the depth-of-field as shallow as possible.
I can usually time my shots and go for what I call the "peak of action". After two scenes I realized this was not going to work and we just were not going to capture the right mix of spontaneous action expreessing a real moment in time. With the child changing expressions and positions every fraction of a second I decided to switch over to a Nikon D4, which can work at 10fps with an almost limitless buffer. I also set it to 3D-autofocus so the focus point could track the child darting about,
Success. As simple as they look to me now, the scenes below, along with many others, would not have been possible without the D4. Nothing like the right tool for the job.