The two main places to congregate on a power cat while underway are the flybridge and the bow area. The main concern for kids in the flybridge is falling down the stairs or falling over and hitting something hard due to the movement of the boat. Obviously, the main concern while underway in the bow area is falling off the front, especially if they’re sitting on the bow seats on the front corners of the boat. Consistent oversight by the adults and good life vests (always on unless in a very secure situation) would be our strategy.
Unfortunately, we witnessed a boat roll over a few years ago by the airport after hitting the reef. There were younger kids on the boat, and everyone was OK, but they said the boat went down within a minute. You never know - so the vests would not be an option for us (just bring comfortable neoprene ones with bright colors).