How easily we can corrupt the essential head-neck-back relationship when picking something up from the floor, note how the infant effortlessly preserves the integrity of his head-neck-back relationship and so keeps optimal strength and flexibility in his spine.
The adult has weakened his spine by kinking at the neck and straining his lower back.
This is not as simple as an analysis of shape, it runs deeper than that, its about the flexibility and tone of the spine. A skilled person can adopt many different shapes, poses or gestures and yet keep a fluid and lengthening quality to the spine. In other words the child could mimic the adult but with an entirely different tone in the spine, shape is often a clue to what is going on but it’s not the whole story. As an example actors trained in the Alexander Technique are often able to adopt what would be a harmful posture in order to play a certain character and yet they are able to avoid much of the stress and strain that you would usually associate with that posture, at least for a short period of time.