Two months ago I took my 6 year old (who is raised in San Francisco therefore may be more accustomed to pedestrian life than, say, the US average) on the train from Naples to Pompeii at 10am, we spent about 4 hours at the site and caught the train back. She was tired at the end but liked it very much, and I was thinking I wanted to go back some time because there was a lot we didn't see.

I didn't expect the vastness and size of the site. Pretty great.