The Argentario Coast has the Maremma's best beaches.
With kilometres of coastline to choose from, you'll find something that suits regardless of whether you're travelling with kids or as a couple.
Most have paid and free sections. You can expect to pay around €10 for two sunbeds, an umbrella and use of the facilities in the paid sections. Most resorts charge per pair of sunbeds and not per person. Most won't rent out just one sunbed though.
As a general rule, beaches 36 and 35-32 near Orbetello and Albinia are the most family friendly with plenty of sand and calm water.
Porto Santo Stefano's beaches aren't all that nice, but they are easily accessible and close to the road.
Porto Ercole's beaches are the most secluded and visually stunning, but aren't great for families as they're mostly rock beaches with choppy water.
The below map indicates all of the coastline's designated beaches, courtesy of the Comune di Monte Argentario:
Beaches on the Argentario Coast