For Formal nite (Monarch of Sea--4-day) what would be appropriate for a 5YO boy? Would just nice slacks, white shirt and tie be ok--I know it's not a tux but frankly he'd have it destroyed!
Fitting him to a tux should be no problem. The tuxedo shops all carry tuxedos in his size because they rent them to ring bearers, etc., for weddings. They would gladly put together a "cruise package" for him.
When you get aboard ship, check at the children's center to see what sort of attire they expect after dinner on the formal evenings. IIRC, the young children have a slumber party after 10:00 PM, and they obviously don't wear tuxes for that. Thus, I'm guessing that the youngsters "dress down" after they eat.