Sometimes kids can be difficult and despite your best efforts, you still aren't sure what to do to make them listen. Clo has the following suggestions:
Give the difficult child warnings out of three.
Usually, it's one child being difficult by instigating situations with another child or children, or it's two children who are feeding off of each other's behavior. If this is the case, make sure to separate the kids with one kid in the middle of the lane, and the other on an outside lane.
Before the lesson beings, have a meeting with the kids to set your expectations for the lesson that day. Remember, you're in charge and they have to follow your rules.
Make sure you focus on the families who are losing out on lesson time due to the distractions that the trouble kid(s) may be causing. Usually when we hear about issues during lessons between students, it's always from a family losing out on lesson time due to the other child's distractions. Swimming lessons are an investment, and losing out on any time can be upsetting for the parents.
 Focusing on the families losing out on lesson time due to the trouble kid(s) may result in the trouble kid(s) getting less instruction overall. It is what it is. Most parents who have difficult kids know their kid is difficult.
Kids are usually on better behavior during Open House (surprisingly so) so we are confident that your Open House will still go ok, despite the presence of any disruptive kids.
There is no point harping on trouble kids and their parents if they don't seem to be making any behavioral improvements. Again—the parents of these kids know their kids can be difficult, and hammering that into them won't help. It's simply better to make the best of the situation you have and ensure that the majority is still having a valuable lesson.