Not a model of perfection but it does raise a few points.
Obviously children have shorter arms, which means that the hand is travelling around the elbow on a smaller circle, making the drop more apparent. When the chin is on the cue, it absolutely necessitates keeping the cue on the same vector throughout the stroke. When the cue stays on the same vector the elbow must drop... its just simple geometry.
If impact is at the lowest point of that circle, on the backstroke the elbow will drop, on the downstroke to impact it will raise and then drop again on the throughstroke. Its subtle but its there.
If the chin is raised slightly off the cue you could theoretically do a stationary elbow position. It sounds very simple, however reality would say otherwise. As the forearm swings back and forth the load on the shoulder changes to maintain that stationary elbow position.