Hi Jen, I bet you're excited, your time is drawing near! You may wear shorts in the buffet and eat dinner there.
Shorts are not allowed in the dining room though. I like to bring a few black separates skirt, slacks, capri's that I can mix and match with a few nice tops, so I can eat in the dining room. I usually pack d/h one or two pair of dockers and a few dress shirts with or without ties and that is sufficient. These things don't take up much room if you want to have a more upscale dining experience.
But make it easy on yourself, do what makes you happy. Your table will be waiting for you in the dining room should you decide to take advantage of it.
Jen if you are hesitant to risk the dining room for reasons previously discussed, I would give it a try and see how it goes. But I would not limit myself to just one night, without really giving it a go. Forgive me, but I cannot recall which ship you'll be on, but if by chance its one of the newer Conquest class ships, I would never want to miss a romantic dinner in the Supper Club. It's worth the extra expense, and on Carnival you will have great activities in the kid's club, so you and d/h can have the guilt free time together you probably wouldn't get to have otherwise (at least thats the case for d/h and I). Everyone is different, but I would encourage you to keep your options open and try everything at least once.