As of today, king salmon have to be at least 20 inches long. That means if you're bringing home fish, they're going to be good size.
You don't need anything. The boat will have all the equipment.
You will need to have a valid fishing license. There's a certain age cutoff though. If you are under certain age, you won't need a license. Check out the California Fishing Regulations.
Just to be on the safe side, take sea sickness pills before you get on the boat. There are special pills for kids. Be sure to read the instructions for both types of pills. Take them the recommended amount of time before you get on the boat.
Always dress warmer than you think you need to be when you head out on the water. Try to dress in layers. If it gets too warm, take off a layer at time.
The kids will get bored if the fishing isn't hot. Be sure to bring snacks. Not just your normal granola bars. I'm talking sugary packed, artificially flavored AND colored goodies. If you don't come home with fish, the kids will have had a good time chowing down.
Last Bit of Advice
To find out how the salmon season is going, watch their Facebook page. They'll post photos and say if they hit their limits for the day.