This is my basic post about allergies..... We travel with a group that combined has allergies to peanuts, treenuts, shellfish, egg whites, chocolate, MSG and liquid dairy. Only had issues with 'user' error, no issues with Carnival. If you have a nut allergy, know that Carnival classifies seeds as nuts.
Each ship seems to do things a little differently. This has been our experiences on the West Coast ships.
If you have an odd allergy contact Special Needs before your cruise. If you have a 'normal' allergy no need to do so.
See the Hostess at embarkation when the Maitre 'd has hours. Hours will be in the FunTmes, usually starting around 1pm. The hostess will take that night's order. Each night the hostess or your waiter will bring you the next day's MDR menu-breakfast, brunch/lunch and dinner. If deciding between two entrees, order both. For brunch don't hesitate to order a breakfast and lunch if you want to eat both meals in the MDR. At ATD, an order slip will be generated and given to your waitstaff when seated. They will confirm the order with you.
Only issue we've ever had was one cruise DD's food arrived before mine.
For room service, they cannot make special for you but they can tell you the ingredients. No special area so there is always the possibility of contamination
Buffet is a minefield. Ask the Hostess or the Head Lido Chef or the Food and Beverage crew in the officer whites. DO NOT ask the line cooks or those serving desserts. They mean well but do not know the full recipes. Cross contamination is always a possibility.
Onshore--- we bring allergy cards. I made our own but you can buy some at selectwisely dot com.
You can only bring off commercially prepared food. We bring cereal bars, boxes of cereal, bottled water, hard candy etc.
Many ports will check. Some will do a physical search, others use dogs. You do not want the cute Labrador to sit next to you. I've had them alert on my Lemonheads.
Kids clubs have a form to fill out. There are some snacks served at times. The younger kids meet up at the dinner buffet certain nights. Ask the counselors what's on the menu.
Gluten Free has a few different things. Cruise Critic has lots of info about GF. Use ‘gluten’ or ‘celiac’ in the search engine. Know there’s at least GF pizza, noodles, bread available.