Pompeii, Herculaneum and Vesuvius veteran here.
Vesuvius is not that steep but conditions can vary so check the forecast and bring waterproofs/sunscreen as appropriate. The city bus from outside Pompeii Scavi will take you most of the way and back for less than 5 euros. The craters are ok but the views across the Amalfi coast are stunning. Bring your binoc's. The highlight for my teen DDs was we could see Vesuvius from our hotel so going there completed the circle.
The historic, ancient city of Pompeii is vast. It is a full sized city, similar in size to Winchester in Hants. But, for me, that is part of what makes it amazing. From the entrance at the old Porta Marina to the amphitheatre is just over 1K. The famous Villa of the Mysteries is 1.2km N of the entrance.
If you're doing Pompeii properly, it will take all day. Bring a picnic/lots of snacks and refillable waterbottles so you can use the original fountains. I enjoy Pompeii because if you relax your mind a little you can really feel what it was like to live there.
You can hire a guide but ...
If your DC is doing Latin (or not), find an old copy of Cambridge Latin Course and read Book 1. It's quick and easy, mostly cartoons with captions and will give them (and you) a great background. I read this with my DD2 by the pool in Seiano a day or two before we went to Pompeii. Caecilius was a real person and you can see the outside of his house ...
Herculaneum is indeed much smaller. I find it's better as a second trip after Pompeii. It's not as well documented so reading CLC book 1 or other info will help more than the tiny plaques at each property. That said, because of the conditions, different things were preserved at Herculaneum which has really enhanced what historians and archeologists think about Pompeii.