I have not yet been on the Grand. Going in July.
Princess does have the soda program, it is actually a sticker that goes on your cruise card. I think it runs $3.95/day.
The cost of 1 can of soda is $1.72.
As for dress code. You usually have 2 formal nights on the cruise. You will see an array of clothing options for formal night.
Ladies wear gowns, cocktail dresses, chiffon pant sets, etc.
Men wear tuxedos, suits. I have seen a dinner jacket and tie.
Some chose not to dress & eat up in Horizon Court (buffet).
The rest of the nights are what they call "smart casual".
Jeans are not permitted.
I am sure that there will be plenty of activities for all ages of your group.
Hope you enjoy