Step-by-step explanation:
Once the scene is safe, one of the pads needs to be placed in the center of the child's chest while the other pad needs to be placed on the center of the child's back. This is because children under the age of 8 years old have not yet developed enough mass. Therefore, adult AED pads have too much voltage which can severely damage the child's heart. Placing the pads in the center and side of the heart can also damage the child's heart even with a lower voltage of the AED pads for children.
Therefore, the best position for the AED pads is in the center chest and center back of the child's body. This is assuming that the child is under the age of 8 and has not begun going through puberty, as such changes like body hair growth can severely interfere with the needed shock.
I have added below a picture of how to perform CPR which is good knowledge to have and can be applied to anyone.