With only 1 day available during your cruise-stop in Ville France, you should definitely concentrate on only one of the 3 cities. Monaco is definitely interesting to visit in many aspects as just looking at the highrise squeezed into a few square kilometres is a sight in itself. The Palace - or rather the entire little village on top by the Palace - is fun to stroll. Being from the US you will enjoy a visit to the inside of the castle due to it's history and all and your little boy will enjoy watching the change of the guards. The little village is truly touristy but ok - or you can concentrate on strolling along the edge of the cliff enjoying the views. The yacht harbour boasting some of the larges yachts I've ever seen will likewise be fun for grown-up as well as kids, and you'll and then top off the day with a drink/icecream at Café de Paris next to the Casino, where watching the people and cars passing by is an experience not to be missed! That's where the Ferraries and Bentleys are "shown off"... We have never bothered to pay the entrace to get inside the Casino, but if that is a major wish, you and and your hubby could switch to go inside while the other stays at Café de Paris enjoying an icecream with the little one, as he is not allowed inside. This area is likewise where you will find the best shopping if money isn't the issue... All the world famous brands are available - and underground you find a very nice 3-story shopping mall very elegantly decorated. So - Monaco will definitely have something for everyone to offer.
Cannes is a most beautiful place - our personal absolute favourite and where we rent a condo when we visit. But - it will take you a while to get there and the drive isn't that interesting and/or by freeway - and the fascinating thing about Cannes is not the Festival Palais but rather it's lovely promenade. The Festival Palais has nothing special to offer - so all you can really do is walking the red carpeted stairs. The beauty of Cannes is it's mile long palm clad promenade boasting turn-of-the-Century hotels and elegant boutiques to the one side and countless restaurants and bars locased below/next to the Promenade offering al fresco dining plus serviced lounge chairs placed on the fine golden sand. This is where one spends the entire day being pampered by young people serving you wine, lunch, snacks while you sunbath and swim and in the evening fine dining and maybe a bit of music under the starts. Next to the Festival Palais you find both the old harbour with loads of fish-restaurants and smaller boats and the newer impressive yacht harbour where you find the unbelievable huge and elegant multistory high yachts where you can watch the staff serving their owners dinner at the top deck... The old town is next to these harbour areas but is tiny and not particular impressive. Along the Promenade you will find ice Booths and carouselles for children but apart from that - and the option of swimming in the ocean from a lovely sandy Beach - there isn't much for a little child to enjoy.
Nice is very close to VilleFrance - and, of course, it has also something to offer although personally I find it way too big a city to be charming, the Promenade not nearly as beautiful as the one in Cannes although much longer and apart from the lovely market selling fresh products and flowers it is more of a real city than a holiday town. The beach is all pebble and doesn't have the same number of al fresco restaurants and good places to lounge and relax for a day on the beach, which you are probably neither looking for, as being on a cruise you have the option to swim in a pool each day, I suppose.
So what I'm trying to say is: Do limit yourself and choose either Monaco or Cannes - but not the both, that will be a rush and I cannot see how you can squeeze them both into the same excursion although using the freeway as they are in the opposite directly of each other and driving along the coastal route takes for ever due to traffic. Monaco will probably be the best for you taking everything into consideration.
PS: By the way. The little town of Ville France where you will be mooring is one of the most picturesque little places on the Riviera boasting a most lovely medieval old town - so don't miss out on strolling it too.
Have a wonderful tour and all the best - PURPLE