This is a real touchy subject and needs to be handled quite carefully. The put-in talk is the best place to communicate the rules and guidelines for your trip. It doesn't hurt to discuss why jack rabbit paddling (racing downstream ahead of the group), tortoise paddling (or endless play time), or walking away from the group without letting someone know affect every one adversely. Describe briefly why you enjoy river running so much and how litter detracts from this experience. Most people think discipline issues are pretty much for children and they are sadly mistaken. Some children have an abundance of energy and a limited attention span, keeping them busy with various activities will certainly cut down on behavior issues.
One major topic to discuss is "Challenge by Choice". You really need to empower the participants to make their own decisions on whether to run certain rapids or not. Let everyone know it is not acceptable to coax or dare someone into running any rapid. Offer to help with the portage when it looks like someone is unsure of themselves as it makes the decision much easier and saves a great deal of time.
On club trips, I typically have a responsible adult for all young children on the trip - a one-to-one ratio. This makes it easy for early removal if necessary. If you are leading a bunch of children with just one helper, you will need to keep them for the entire trip.
When working with a number of young children, you personally have to be on your best behavior. The trip leader sets the example starting with time management and coolness/patience under pressure. Maintain this example by sticking to schedules so everyone has clear expectations. Avoid shouting (or using your safety whistle) as it leaves the impression you have lost control. Keep children busy on various activities and reward with earned praise. A small example is finding a spot in the river with several eddies that are easy to attain and running eddy drills. Another really fun one is throw rope practice in a deep and wavy current with a pool below. You can structure the throw rope practice to reinforce team work. Wading wedge crossings also build team work - just pick a safe area to do this exercise. I highly recommend having extra adults when leading a bunch of young children as it provides more options and more good examples as well. Take bullying very seriously - no tolerance whatsoever. The same goes for any taunting. You can't leave a young child on their own but you can isolate them by pairing with an adult for the remainder of the trip. Basically you rearrange your paddling partners and keep the two quite separated. This topic should be discussed as part of the challenge by choice discussion and made clear we have no tolerance for this behavior. Participants that bully others are no longer welcome on your future trips and will be isolated immediately.
Scouting organizations across the country deal with situations like these all the time and have dealt with discipline in a number of different ways as you would expect. Here are a few articles with some tips:
