I'm directing (and animating) a TV commercial that will utilize pixilation. Similar to this down shooting style.
The biggest (and hardest) difference is that the client really insisted on having a 3-4 year old boy and 8 yr old girl.
I started trying to brain storm ways that I could using "rigging" to gently encourage them to stay their different positions.
So far I've though of a small foam wedge for under their head/neck for comfort and help keeping their heads in the right spot.
I thought I'd share with you guys and hopefully find a seasoned pixilation veteran, or at least some creative ideas for helping the young kids stay still.
Drug them unconscious, then you can move them like puppets?
Should be possible with an 8 year old (getting them to stay still, not drugging them!). But I imagine you could only get a 4 year old to cooperate in fairly short bursts. Fortunately, it doesn't have to look smooth and perfect, it it looked like real motion there wouldn't be much point, it's the slight jerkiness that adds the charm. So if you had to take a break, then put them back where they were in the last frame, you could get away with it being slightly out.
If they can't hold still and move (or be moved) the right amount for long enough, maybe try getting them to move in slow motion, and video it. Then change the speed so you remove most of the frames, to get a more animated style. It wouldn't take as long as shooting stills so would improve the odds of getting something done before they get too restless.
That's all I can think of - I don't do pixilation. Or have much to do with kids either. Puppets don't get cranky, and are really good at staying where you put them. Good luck with it!
Have them move as slowly as they can, and then take frames faster when they move too quickly. This will balance out the speed-of-motion-to-speed-of-capture ratio.
If you really can't get them to stay still, try using a lower frame rate. 6 FPS works well for pixilation if the movements are fairly large. The unnatural cadence can also be rather comical.
Edit: Some of this is an offshoot of what Nick suggested.