In football, when subjects are running directly towards me, then the AI Servo cant keep up, even if I half press it to focus and full press the shutter button in a split second.
Shooting MotoGP and F1 I have found that the longer I let the camera "see" the subject, the better the result. It seems to need time to figure out the approach speed and know what refocusing it needs to be doing. That's with my old 30D and Eos-3 as well as my current 5D2. I can only imagine how difficult it is doing those calculations for a toddler who is hardly working at a constant speed. I'd be very tempted to use another method, not AI Servo, in that case.