Hi Kim (glory bound), and Welcome to Cruise-Chat!
As has been noted the dress code has been relaxed a great deal for the main dining room, even on "cruise elegant" nights. For children of those ages causal shirts (non-T's) will be fine, along with pants, or non-cut off shorts. So no need to worry.
I sorta doubt with you being from Newfoundland you'll want to eat a lot of fish on the cruise. Just in case though, the Glory is one of the Carnival ships with a Fish n'Chips outlet in the Casual Lido buffet restaurant. It's usually easy to get a table as most people don't realize it's there as it is one deck up within the restaurant. Even if you get your food from the regular buffet, or another specialty outlet, it is a great place to sit and eat.
I would go to the Main Dining Room though for dinner!