We don't speak Italian but muddled our way through to take the train to Pompeii from the Naples port. Here are my tips:
From the Naples cruise ship terminal at Stazion Maritima Molo Angioina (for Celebrity cruise line), we took a bus (be careful since one will only take you to the airport) to the Napoli Centrale Train Station around 7:30 am. You will need Euros or a credit card. Downstairs is the Napoli Garibaldi station with the Circumvesuviana train to Pompei Scavi station at 8:26 am (be careful there is a Pompeii train station which is 9 km away and had more trains going to it) which is about a 30 minute ride, but the train was 20 minutes late, so it took us about an hour to get to Pompeii. It is only two stops away plus one where the train does not open the doors. Another older train came by but luckily a station attendant told us it was the wrong train. Trainline is a good app for seeing train schedules and buying tickets and we bought our tickets a few days before.
At the Pompei Scavi train station, there is a bathroom for 1 Euro (no seats on the toilet) with an attendant. It is a 2 minute or less walk, basically across the street and down a little to the Porta Marina entrance gate where we were to meet the Walks of Italy Complete Pompeii Skip the Line Tour with Archaeologist Guide tour guide.
The Pompeii tour is definitely not accessible for people who need wheelchairs, walkers, or have trouble with walking on uneven surfaces or climbing steps. The stone streets in Pompei are not smooth but slightly uneven, and you have to climb down or up at times or cross between blocks on stone steps. There are wagon tracks cut into the stone as well.
After our guide got us our tickets around 10:40 am, we were standing, walking, climbing until after 2:00 pm where we finished the tour at a place where we could eat our own snack/drink or smoke. No smoking is allowed most places. There were bathrooms downstairs in the gymnasium training area.
You need to bring your own bottle of water, and there are running fountains along the streets where the guide let us know where we could refill out bottle. I just used my hand as a cup to sample "Pompeii" water.
After the tour ended, we walked around some more for another hour, then took the 3 pm train back. It was packed standing room only with people with suitcases, and I am glad we left when we did to beat any rush hour since it is possible other trains would be too full to get on and we had a 6:30 pm be back on-board cruise time.
We took a taxi back to the ship. It was to the right when you exit the train station. To the left is where they drop people off and you cannot get a taxi on that side. The taxi ride back to the cruise terminal was 20 Euro. The meter said around 11 Euro but there is a cruise port surcharge we were told.
On the taxi, I saw a sign for100 Euros to Pompeii for a roundtrip with 2 hours to see the site with 22.50 Euro for each additional hour of extra time. It will be likely more with 4 passengers, but it give you a comparison. The Express train for the two of us was 30 Euros, so perhaps a taxi might be easier for your family.