I spent a week in Rome last week with my wife and 2 kids (ages 9 and 14). I had been to Pompeii 25 years ago, but my wife had never seen it (or Rome), and since we had such a long stay in Rome, we thought it would be a good idea to take a day trip out of the city. Thanks to this forum and it's valuable information, along with Rick's Rome book, we had a very enjoyable and successful day.
We booked Trenitalia tickets departing from Termini at 8am. I booked these about a month in advance and it saved me over 100 euro compared to if I waited until the week of the trip to book. We had an airbnb just across the river from Castel Sant'Angelo in a great neighborhood, so we were out early to get a cab from the stand at Piazza Navona. Arrived at Termini around 7:45 with plenty of time to find our track and reserved train car. We booked the 'premium' train tickets, which is a small step up from the standard, but below the business class. Comfy, spacious, and 4 seats facing each other with a table in the middle. Complimentary snacks and water distributed, and we were next to a cafe car that had a very friendly man working who made a great cappuccino.
Direct train from Termini to Napoli Centrale was 1 hour 13 minutes, so we arrived at 9:13 and had a bit of time to kill before our Circumvesuviana train that departed around 9:55. This was good because it was a bit of a long walk from one train platform to the other, and it gave us time to let the kids use a (gross) restroom and offer them food from somewhere in the station.
The train to Pompeii Scavi was around 30 minutes and it drops you off literally right outside of the entrance to the Pompeii site. I used Ricks book to reserve a tour with Antonio Somma (cost 200 euro for 2 hours). He was extremely good with email communication after reserving, and closer to our date, we established contact on WhatsApp. He assigned a great tour guide to us, Veronica. She met us right outside the station and we commenced (WE had to buy our entry tickets online ahead of time, and I brought the printed copies with us). Veronica was great, I think she said she's been doing tours there for 20 years. She seemed to know everyone, employees and other guides.
The 2 hour tour was perfect for us, we covered a number of interesting things, but it was not too much walking where it would have worn someone out. My wife and kids really enjoyed her and her tour. My first visit 25 years ago, I self guided myself using a tour book, and that was fun for me because I could go at my own pace and really cover a lot of ground. But after 2 hours, my family was satisfied, and hungry. Victoria said she could escort us to the exit, or we could stay to explore or eat. We wanted to get some food, so we said goodbye and headed to the restaurant/snack bar up on a hill in a newer (Not 2000 years old) white building. The walk up to this took us over a catwalk that was covered by roofing, and this led us above a part of the city that is currently being excavated. This was really great to look down and see real history being unearthed right now. You could see the archeologists doing their gentle work, and a lot of stuff that was being unearthed and catalogued.
Snacks/food were better than expected and not too expensive. We called it a day after that and headed to the train back to Naples where we planned to kill a bit of time getting some EXCELLENT Neapolitan pizza at Pizzeria Trianon de Ciro. We got there just after 2:30pm and there were only about 6 other people there. Pizza was great, wine was excellent. The walk from the train station was not too far, and a nice look at gritty Naples. We killed a bit of time after pizza wandering the city and were on a train back to Termini around 4:50, arriving in Rome around 6:05.
It was the perfect length for a day trip. If I was solo, I would have made it longer and spent more time in the ruins.