Younger kids can run riot at Parco Giochi Bim Bum Bam. They'll sleep soundly after an afternoon of climbing on and swinging off the equipment.
To explore this outdoor play area, make tracks to Viareggio. You'll find plenty of other things to see in the area, including Massaciuccoli Lake, Pontile di Lido di Camaiore, and La Cittadella del Carnevale.