Etruscan Coast

I’ve posted this recommendation a number of times, and we visited there both in 2021 and 2022. Castagneto Carducci is a remarkable hilltown less than 4 miles inland from the coast, and about 5 miles south of Bolgheri, another cute hilltown, and a major wine center. Right on the coast is Donoratico, with a great beach

All these are a short distance up the coast from many ancient Etruscan sights. Both times we stayed at a farmhouse with a big garden (you’re welcome to help yourselves to produce in the garden), Chicca’s Casa Toscana. There are two apartments on the second floor of her house. Chicca teaches cooking classes, too. Here’s the Website: https://cookingintuscany.cc/casa-toscana/

Month of the year?

It's mostly a matter of East Coast-er opinion, but I'll observe that the United States has some of the best ocean beaches in the world. I'd make sure that your daughter's interest is specifically about Italy (say, from watching Hotel Portofino on PBS or something ... ), rather than just a reaction to living in a landlocked state. (I did live in Cleveland for two years.)

You did not give your full itinerary, so we don't know the overall cost (I mean time, of course) of indulging her request.

I fairness, while I have been to Sardinia (and of course, Venice), I have not been to the Italian Riviera. But I've been to plenty of windswept, cold, if huge, beaches, like Schveningen, St. Malo, Ostend, Normandy, and so on. The Mediterranean is not as clean as, for example, the Adriatic. So I don't swim in it. Besides, the pebbled beaches in Positano are two small and uncomfortable, and the water too cold. (No, you didn't mention the Amalfi Coast.)

It depends on your commitment to art history, but I don't count three days in Florence as a visit to Tuscany. You can barely see Florence (and Fiesole) in three days. You certainly don't want to use the car just to get to the coast. It takes a surprising amount of time to visit each hilltop town in Tuscany. And if you pick them right, every town is different.

This is not a substitute for her request, but as a view of Living Like Locals, I was very struck by places like Castiglione del Lago. There are other lakes, too, in Tuscany and Umbria where you'll find a modest beach, in season, jammed with locals who have no desire to go to the ocean. And Cast. del Lago is nice historic-Tuscany stone village, with an estate/castle as well.

The part of the Tyrrhenian coast that you can reasonably reach from Volterra is not dramatic in the least. Your granddaughter can discover it next trip. Better to turn her toward the miracle of the Renaissance and learning about and imagining Tuscany's differing landscapes and cities peopled by peasants, artists, and Medici with their cavalieri. The Castagneto Carducci recommendation is worthy and the nearby town of Bolgheri is memorable to wine lovers. Also nearby to the north is San Miniato with some interesting attractions and views. https://www.wanderingitaly.com/places/san-miniato.html

There are beautiful places everywhere in Italy, including on the Etruscan Coast, and I agree the area of Castagneto carducci, Bolgheri is one. You don't mention what time of the year you're going.
Baratti and Populonia make a nice half day trip, if you don't do the hike we did here, which is good in the spring: https://slowtravelitalyspain.blogspot.com/2021/07/costa-degli-etruschi-may-2021-baratti.html
To eat some fish and walk along the beach, San Vincenzo is nice: the beach behind the Parco Costiera di Rimigliano/Principessa is very natural
https://slowtravelitalyspain.blogspot.com/2021/07/costa-degli-etruschi-may-2021-campiglia.html
Campiglia Marittima is also a beautiful village - be sure to visit the cemetery too, and "Il Goccetto Vino e Taglieri" which also do a small but splendid range of local dishes is excellent.
So, I think you could combine pleasing your granddaughter with seeing some splendid places and different landscapes usually outside the main destinations for foreign tourists.
I'm happy to provide more info about places. The area is one of my favourites.

I spent a day hiking at the regional park next to Albarese, but it’s a couple hours from Volterra so might be a little out of the way for your trip. But for the future, it’s through some forested area with ruins of a monastery, then goes over the coast with great views of the sea, then drips down to the coast and through a grove of umbrella pines. Southern Tuscany is a beautiful area that I’d love to get back to so hope you can find something off the beaten path.