Which principle states that a greater-than-normal stress load on the body is needed for the body to adjust to training?
a. reversibility
b. progression
c. individualization
d. overload

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. 

Answerfact;