A shame that happened. I expect, from reading the newspaper's account that this might be a classic example of "diffussion of responsibility."
This happens sometimes when several people are involved, each thinking the other is taking care of business. For example, one day at the Magic Kingdom my wife, my son, and his son (my grandchild) and I, were enjoying a day at the park, and took several photos during the day. The four of us were taking turns carrying the stuff we had taken into the park. But, just after we exited the park, we realized that none of us had the camera. Each of us thought someone else was taking care of the camera.
It was an expensive camera, and it was never found.
On all subsequent visits to The World, each member of our party has specific items to be responsible for while visiting a park, and we've never lost anything since.
I know there's a big differance between a camera and a child; but the reason for the loss might have been pretty much the same.