Hi, Carpooler ...
If you cruise on over to the First Time Cruisers area you will probably find out more than you ever needed to know about luggage !!!
I think the 6 YO son would be fine with nice khakis and what I would call a "blazer shirt" .. long sleeved and well-pressed ... the 14 YO might be better of with "the uniform" of blue blazer, shirt, tie, and khakis. Don't worry about their shoes. Their feet will be under the table.
The birth certificates MUST have the official "raised seal" from the issuing office. You have plenty of time, you might want to get passports. For adults, they are good for ten years, for kids, five years. But you still need the raised seal birth certificates to get passports.
Keep asking questions, we're here to answer.
pamda -- CM First-Time Cruisers Editor