Read further, he supposedly racked the slide after removing the clip. Meaning that it should have been unloaded.
Actually, on a lot of guns you can pull back the slide to reveal the chamber without ejecting a bullet. Perhaps instead of racking the slide forcefully, he just pulled it back to show the kids and aked them if there was a round in the chamber, and then took their word for it when they said, "No, Officer Friendly!"
Which brings to mind the question, did the little kiddies set him up?