Make sure Carnival knows that the four of you want to be seated together....then if for some reason you find that you aren't at the same table at the same time in the same dining room (info on sail and sign card) contact the Maitre'd and ask if you can change your seating so that you will all be together. Although you don't say which ship you will be going on most have tables for four....if you aren't placed at one of those then it would likely be at a larger table with other people.
I travled with another family also we booked at seperate times through different travel agts. we just made sure that each travel agent had our booking #'s to ensure a cross refrence. Not only did Carnival sit us together they put us in cabins right next to each other so that made it convient to get together each day