MichelleB

We visited these ports when my kids were 7 and 4. In Costa Maya we did an excursion with Native Choice that included the ruins and then we had lunch with a Mayan family. In Cozumel we went to Chankanaab beach park which had a little bit of everything. The kids and adults all enjoyed both excursions.

I second the hamper or laundry bag to contain dirty clothes. You may also want a white noise machine/app as it can be a challenge for 4 people to sleep in a tiny room. 

We booked private transfers through Civitavecchia Shuttle Express. It will be much more convenient to have door to door transportation and the price was reasonable. We have not used them before, but they have good reviews and the communication through the booking process has been great. 
From what I've read the train is a good and economical transportation option. We decided against it because the train station in Civitavecchia requires a decently long walk with luggage plus a bus ride into port or a cab from the train station to the ship. We also preferred to stay near the Pantheon in Rome which is not convenient for train/metro access. I figured that the private transfers didn't end up being that much more expensive for our group and it will make our travel days much easier. 

As long as you're not being wasteful then it's not a problem at all.  We've never encountered an issue and our wait staff has often brought extra food unprompted if they think we might really like something that wasn't ordered, may bring 2 items if we were indecisive, etc. My 8 year old is a big eater and often had a 2nd appetizer or 2 entrees and the wait staff was always happy to bring it.