What category of exercise builds cardio respiratory endurance

Treadmill, eleptical, bike or any other cardio machines

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. 

A) stable development i believe