I've stayed in Sorento and done both on the same day. It's a lot of walking so you may end up carrying a 6 year old. It was also a full day trip. We left Sorento early and first went to Herculaneum, then Pompeii, and didn't get back until it was dark. Highly recommended.

Also keep in mind that the train (Sorento<->Naples) is rather (in)famous for pick pockets, though we didn't have any issues.