Porto Carras is beautiful--you will find lots of veg. options in the salad and appetizer sections of menus--grilled vegetables, dips, bread, salads, cheeses, pickled items. For the entrees, pastas and rice dishes will be your primary choices. Not sure if you do seafood, which would widen your options quite a bit.
With two days in Athens and your desire to visit the Parthenon and Plaka, stay in a nicer hotel in Plaka, with a pool and rooftop restaurant/bar that has views of the Parthenon by night. This will be most relaxing with kids. There are plenty of options if you look at the various hotel suggestions online.
The temps will be a range--like October in the southern U.S. It could be as hot as summer (but still cooler at night) or it could be on the chilly side (50s-60s).