No one is clutching their pearls. I might say that the fact that you posed the question meant that you had doubt in your mind. The thought of stretching the courtesy of one cabin priority boarding to ALL cabins having priority boarding is, well, a bit entitled, is it not?
By asking someone at the port (who is NOT an MSC employee) to honor the request you are putting them in an awkward situation because they don't know if you are going to snap on them and start getting all "well, I'm Diamond" ...
No one is sniffing at Bella category; it's a fabulous option. However, with categories of service on MSC as well as levels of status there comes different services. So if you are in Fantastica does that mean the other cabins you are traveling with can get room service breakfast without charge?
If you are all linked for dinner take that as a win. As Far as boarding, for the extra 10 minutes it might take you might to stick with your kids vs putting everyone in an odd situation to start out the cruise.