What is the white opaque "moon" often found at the proximal end of a fingernail called?

The white lunar segment is called a "cuticle". It only real function is to help (minutely) with support and covering the nail.

If it's a smaller child or infant, place one on the middle of the chest and on the child's back. (if it is a bigger child with enough room for two, place one on the upper right/your left of the chest, and one on the lower left/your right) the pads should never touch each other.