Child's pose or balasana is a bend, which stretches and releases the spine and lower back. You can use this pose as a warm-up pose, a resting pose or a counter pose for a back bend.
We usually don't breathe consciously and fully into the back of the torso. Balasana provides us with an excellent opportunity to do just that. Imagine that each inhalation is "doming" the back torso toward the ceiling, lengthening and widening the spine. Then with each exhalation release the torso a little more deeply into the fold.