I have this same question. We will be in Naples/Pompeii on a Saturday in July. Everything I'm reading suggests that weekends in the summer can have really long lines, an hour or more. We only have a few hours at Pompeii, so I really don't want to spend so long standing in line.
I'm almost tempted to just buy my son an adult ticket to guarantee skip-the-line entry. The extra hour of time in the park would be worth the cost to me. Does anyone know if there is any reason they wouldn't honor an adult ticket for a child?