You might want to think about using a different focussing screen - e.g. with my mamiya RZ the Type E one (Rangefinder Spot/Microprism) instead of the standard matte - that can be a great aid in the right circumstances, together with being prepared to move yourself rather than your camera as suggested somewhere above - though I prefer the matte screen for other types of work. It's amazing how you CAN get the focus quickly with MF and kids, although I have to say you do have to be prepared to miss some shots aswell. But personally, I have never felt the need to use autofocus with children, even with 35mm. What you gain with some shots missed with manual focus you can lose with the autofocus making the wrong 'decisions'.