Thanks, everyone, for the watch schedules! Sailing as a couple alone, we used 4 on/4 off until we got into some weather and then had to go to 2 on/2 off because we had to muscle it with heavy following seas that overwhelmed the autopilot and would slew her aft around if the helmsman wasn't careful. I like the 3 on/3off at night, I think that might work better -- 4 hours can be a very long time in the wee hours.
We're going back to sea with our son, now, who is old enough to stand daylight watches, we will probably use him for dogwatches to stagger our own schedules, and thanks for the idea. I would not want to put him on more than 2H at a time, and better it be at a time when one parent is awake below, cooking or doing other useful stuff. It's a little scary, but it's a labor of love.