We have cruised 6 times and would do all except Florence by ourselves, although others would say Florence DIY is OK. For yourselves as first time cruisers my advice would be to do Villefrance and Ajaccio by yourselves, but take an organised trip for both Naples and Livorno.
Cannot comment on La Goulette as we have never been, but a very high proportion of those who have would advise you stay on board and treat this port call as a "day at sea". The locals are extremely intimidating. From personal experience in Casablanca, which apparently is not as intimidating as La G, if your 14 year old is a daughter think very carefully. Our daughter was that age in Casablanca and all the local males, men old and young and the boys ogle at females as if they had never seen one before! This is an extremely frighteningly unpleasant experience for a child.
If you have never been in Villefrance or Nice, it is a lovely experience just to explore them both. Even if you wanted to go to Monte Carlo it is very simple, and cheap, by train from Villefrance. In Ajaccio simply explore the town on foot, and there is a trolley road train which takes you round, and out the town for a couple of miles and is very pleasant for adults and children.
One final thing, a health warning. Like us and many, many others you are at extreme risk of becoming hooked on cruising!!