At My job Ymca?

I'm posting here because I have an issue I currently work as a lifeguard for the ymca inside pool. While I'm on the chair besides the pool guarding I have notice twice that two youngsters are pulling a prank on me by playing a watch trick just like a wallet trick they have tie a gold watch to a thin rope put it in the bottom of my baresoles and then when I'm going to get it with my foot they pull it away this has happen to 2 other lifeguards. What should I do if next time they do this and I or the 2 lifeguards stomp on the watch if they do it again should we discipline them or take the watch from them.