We have the following rules when under way.
Everyone is assigned a life vest and a buddy partner. Small children where assigned an adult as well.
No one was allowed on the pilot house and salon roof. If they did had to let me know and have a reason. They have to wear a life vest. I closed off blocked the stairs and ladder.
No one was allowed on the front deck without life vest and or their buddy. Children also had an adult.
Children had to be inside the boat, if out side on the stern deck and behind the Portuguese bridge a life vest and an adult.
The biggest safety is to provide something to entertain them inside, movie, games, puzzle so they would stay inside.
We usually started out early in the morning, between 6 and 7, so they slept and had breakfast the majority of the morning, and tried to keep the cruises short and hopefully before noon. Even adults get restless after 4 hours.
Keep in mind actual cruising is boring to most children and adults, so keep cruises short, while they sleep, and keep them intertained. Heck even I and my wife thing cruising is boring a high% of the time.