It only takes a matter of minutes for a child to get injured. I work in a B&N. One night a little girl (6-8 yrs old) was left alone in our children's dept. She was trying to reach a book that was too high for her and pulled over an extremely heavy book display. If this display had fallen on her, she would have had serious injuries. Her father had no idea what she was doing because he was off shopping. We had to track him down.
We want our customers to be happy and feel safe in our stores, but it is not our responsibility to make sure your child is safe. I personally do not want the liability of watching anyone elses child except my own. I know what anguish parents feel during the times we have had to call Code Adams in our store. It isn't even my child missing and my heart aches. Fortunately the child has always been located.
Then there is the costs that B&N incurrs while parents are not watching. All those pages torn out, stickers removed, pop-up books torn apart, sometimes while the parent is sitting right there so absorbed that they have no idea what their child is doing. Of course there is never an offer to buy the product, which is now unsaleable. We find damaged product hidden (guilt but not enough to buy it) and put back on the shelf for the next paying customer to find.
Your child may be a paying customer but in the big picture, he can still be a paying customer, safely and responsibly by your side.