Now I'm going to put this in another direction...dressy night for kids?
I've found that little kids -- especially boys -- can dress down a lot. My 8 y.o. has cruised lots and he usually wears khakis and dress shirt -- no jacket b/c he hates getting dressed up! And it's always been fine.
For more details on family cruising, check out Cruisemates.com's main family page with lots of articles giving practical advice at:
Family Cruises; CruiseMates Guide
Family Cruise Editor