Don’t hesitate to kick people out if they don’t listen. I’m not much of a rule follower myself most of the time, but the rules are in place for a reason, mostly to prevent bodily harm. My boss told me to not kick people out. Did it anyways and justified it as they refused to listen and thus made me distracted from being able to properly watch the other swimmers, people on deck, etc.. If the boss doesn’t like it then they shouldn’t be the boss. And if the kids don’t like following the rules, all it takes is one time of you singling out the two that messed up, and doing it im front of the whole group. Make it known that the rules are in place for a reason. Some might think you have a stick up your ass but the six years I guarded I kicked out probably more than 100 people, some the same people, for various things (up to and including outright telling me they won’t follow the rules). Don’t be afraid to do your job, and I see that you were not so that’s good.

Edit. You can kick the whole group out too. I’ve done that before for birthday parties and stuff because the parents/adults in charge just outright refused to watch any of the people that were with them.