Beach in Emilia-Romagna with cabanas?

Can't help you with specifics, but you'll likely find what both you and your daughter crave in Rimini. However, if you'll be in Florence on your way to Venice, Rimini seems easy but wanting to take in Parma, Modena, and maybe Mantova would add some complexity to your itinerary (much of the major train movement in Tuscany and beyond, revolves around Bologna.)

Ferrara would be an excellent stop to consider!

If you did land in Mantova, a plate of Cotechino with lentils and a side of Pear Mostarda would be a pretty nice typical meal of the Emilia Romagna.

Venice has the Lido beach area.
You could get up to Venice and get settled there, and your daughter could be on the beach at Lido while you do something else in Venice.
Or you can walk around and explore Lido while she sunbathes!

https://lifeinitaly.com/lido-of-venice/?msclkid=d345f496cefd11ec9f33c37a715aa50e

http://www.ogvenice.com/beaches.html#:~:text=The%20Venice%20Lido%20is%20one%20of%20the%20islands,from%20central%20Venice%20and%20easily%20crossed%20on%20foot.?msclkid=1eec96c3cefe11ecb0a145b3810e447a

Use Modena as a base to visit both Mantua and Parma. One hour and 35 min away from Modena by train respectively. No sense changing hotels.
The Romagna Coast would be a good choice for your daughter. The beach and the water is nothing to write home about, but it’s a vibrant place for the young crowd and it’s known for being the entertainment capital of Italy. Rimini is easier to reach since there are trains getting there from Modena without change (2 hours). However I prefer the quaintness of Cesenatico or Cervia ( 2,5 hours with a train change in Bologna).
No matter what however, if your final destination is Venice, the Romagna Coast is a detour. A compromise with your daughter might be to find accommodations at Venezia Lido, as suggested above. The Lido is not as vibrant and full of entertainment like the Romagna Coast, but it is just a short hop from Venice’s historical center with a vaporetto. So while you visit churches and museums your daughter can stay in a cabana at the beach. Beach comcessions offer cabanas, umbrellas, chez for rent pretty much everywhere on the Italian coast. Some luxury hotels, like the Excelsior at the Lido have their own private beach with cabanas and all.

Although I don't have a lot of experience, beaches in Europe don't tend to be as "good" as the best beaches in the U.S. and in the Caribbean. Does your daughter want to imagine she is with Picasso and other artistes on the Riviera? (I'm not mocking her, I'm asking WHY she wants to do something she can easily do at home while overseas. It's not like you live in Iowa.)

A more "local" and "Italian" experience might be a town on a large lake, that has a beach used mainly by the residents. There are a few of these in Tuscany, like Castiglione del Lago, for example. But I don't know about E.R. I personally can't imagine sitting on a beach in Venice when I could be looking at art that never travels to museums.

I have been to few European beaches, like Sitges, Spain, and Ostend, Belgium. Each has its own unique characteristics, but none is as nice as (one childhood memory of mine) the semi-public Tobay Beach on the south shore of Long Island. I wanted to visit the Casino in Ostend, but alas, it was only open at night. (I wanted to see a bronze statue of Marvin Gaye there, not to gamble .... ) The most interesting thing I saw on the wide, flat, boring beach at Sitges (besides undressed sunbathers) was the plethora of fake U.S.-branded tee shirts for sale, including a lumberyard I used on the West Side of Manhattan.

Cabanas aren't popular in Emilia-Romagna. Maybe there are some ones somewhere, but I have not in mind them.
For your daughter it depends by what she want to do. The coast of Romagna is perfect to relax and have fun on the beach. For a young woman central Riccione area or Marina Centro in Rimini are probably the best location where to spend time on the beach. Are places often crowded by young people and with a lot of facilities.
Marina Centro in Rimini could be even a place where you can leave her and go to explore the historical center: Rimini in fact is not only beaches, but even art and history.

Another solution could be Milano Marittima, but less easy to be reached by public transport. On the other way, Milano Marittima could be even a good base to stay for a couple of nights and explore cities like Ferrara, Ravenna, Cesena, Forlì...

I agree that the Adriatic coast in the Romagna and Veneto regions (basically the Po Valley) is kind of ugly. I disagree with Tim’s statement that is true for all beaches in Europe. Sardinia and the islands in general have nothing to envy from US coastal areas.