The is the region of the eye where photoreceptors are most highly concentrated.

Fovea. 

The fovea region is where most photoreceptors are located. This is because most of the rods and cons are located in this area of the eye. Moreover, this is also where transduction happens. The part where the ambiguous stimuli which is light is interpreted to be a meeaningful information. 

Cotton with iron probably

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.