I will second what Donna said about using a taxi (or a car with driver, called NCC here) that can take you to where you want to go - you don't have to drive nor worry about how much you drink.
This for the further to reach places, if the walk is too much for your parents.
An alternative is to join a small group tour that might look appealing, as they take you all the way into the estate - take a look here:
http://www.partner.viator.com/en/136...ti-Tuscany/ttd
Aside from these, you are along the road where the 365 buses between Florence and Greve, so you catch that and go to the wine shops found along the road -- now, all of them allow for wine tastings. BUt if you want lunch too, then the estates are generally away from the road up the hill.
JUST ASK at the wine shop! It might be likely that they won't mind taking your parents up the hill, to save them the walk :-).
Two more possibilities along this Chiantigiana road:
Castello di Uzzano, right at the entrance into Greve: agricolauzzano.com
Villa Calcinaia -
http://www.conticapponi.it/en/guided_tours/
Contact them to ask whether they can accommodate your requests, regardless of what they have on their site.
Now about lunch and non-vegetarian choices. Just ask as well whether they can offer alternatives, otherwise they most always include cold cuts such as salame, finocchio and prosciutto, as it is a Tuscan specialty and they assume you want to taste Tuscan products with their wine.
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