My suggestion is to not listen to them. I'm cruising Costa for the first time in November and despite everything I've read here, am really looking forward to it. Understand that Costa is a European line and that Europeans do things differently than Americans.
I've heard many complaints from people regarding the food, the service and the fellow passengers. It's probably no different than people I know that have gone to Europe and complained that the food in Italy isn't like it is at Olive Garden, that the service in Spain is slow and disorganized, and that the French are rude and unfriendly.
If you want Italian food like they have at Olive Garden, don't go to Italy or go on an Italian cruise line. They do not eat what we eat!
If you want good service, be able to communicate with your waiter. Since Costa caters primarily to Europeans, the waiters shouldn't have to speak English fluently to accomodate the small percentage of Americans and Brits on their European cruises.
The French are warm and wonderful people but like most Europeans, take offense to Americans who EXPECT them to speak English and make no apologies for not being able to speak their language.
Kudos to you for taking your children on this wonderful adventure. If none of you know Italian, it would be helpful to either take a class or get a Berlitz course and do it at home. Being able to communicate with people makes all the difference when traveling abroad. I've been on 18 cruises and have NEVER been on a bad one. Take a good attitude and a sense of adventure and you'll have a great time. If you want familiarity and to be surrounded by Americans, take an American line instead.