There are a number of beachs on the coast South of Livorno to Piombino. Have a look at Marina de Cecina which is located about 2 hours from San G. It has alot of beach rental facilities which would help with children.

www.viamichelin.com

Enjoy!!

Marina di Cecina is an option, another good one could be Rimigliano beach, which is south of San Vincenzo, on the coast between Marina di Cecina and Piombino. It is family-friendly, safe, clean and unexpensive. Viamichelin can help here as well.

We just returned from a week in San Gimignano and I would strongly recommend NOT going to Marina de Cecina for a beach day out as we unfortunately did. Firstly, the drive there from San Gimignano is very difficult, the roads are very winding and narrow so what looks like a short drive actually takes 1.5 hours. Secondly, when you get there, the beach is not that great, a narrow strip of dirty grey sand and murky water. All that would probably be acceptable if you felt welcome when you got there. However, the beach there has been walled off by provate beaches, and although the vast majority of the sunbeds and umbrellas were not being used when we were there, no one would rent us one as they were "all booked for the season" by Italian families. In fact, they would not even let us sit on the sand with our own beach towels unless we went to a narrow stretch (10' wide) of horribly overcrowded beach between the private beach areas. We have travelled extensively in Europe before and never run into this situation where people with money in their hands to rent a sunbed could not do so. Probably the final straw would be that when my duaghter wanted to use the bathroom at the cafe we had had lunch at earlier in the day, the owner refused to let her have the key! My advice would be to avoid Marina de Cecina at all costs, the locals clearly do not want to make tourists feel welcome.