Coastal town close to Florence?

I would take them to Senigallia. It has a long, wonderful golden sand beach, with lots of hotels and restaurants lining the beach, with beach toys, volleyball nets, etc. It's on the Adriatic coast, so there are no big waves ... you can wade out really far in the water before it gets deep. We were there in October and wished we had our granddaughters with us. Our favorite restaurant was called La Tartana. You can sit at a table on the sand, enjoying a glass of wine (under an umbrella, if you choose), while the kids are playing in the water. For dinner, we would have large servings of homemade pasta with fresh seafood, for about 10 euros. Senigallia is in Le Marche, about 3 hours via train from either Venice or Florence, but well worth the detour. It's an interesting typical Italian town, with a 17th palazzo and 14th century castle ... wonderful daily fruit and vegetable markets. And then just a few blocks away, there you are on the beach! I hear that Senigallia can get really crowded in August, but if you are going in June or July, it would be perfect.

Viareggio is the only real choice. It's lively, good beaches, lots to do both in town and the surrounding area, good train service. On the downside, it is a bit of a trip to Florence (about an hour, I'd guess), it's not old, and your friends won't be jealous.