Nature Play claim that a child's movement in the first two years helps them grow, specifically a rocking motion like in the womb. The link to the supposed scientific source is now dead. Is there any truth to this claim?
From their website
Have you ever known a baby or child to lay or sit still when they are not asleep? Movement and motion both stimulate the brain to grow. In the first two years of life a child's brain will grow from 25% of its adult size to 80% of its final size. Science has identified that movement is an important key to making this growth happen. More specifically, it is the rocking motion of walking that does it, the same motion they 'grew in' in the womb.4 So there is a good reason why children can't sit still and like to be rocked, they need it to grow, it is good for them.