from the beach , particularly from the neighbourhood of the three playground enclosures -'joie de vivre ' , 'helio ' and 'azur ' - came a twitter of children 's cries that waxed and waned with the thrill of their games and , farther out , on the firm sand left by the now distant sea , the shrill whistle of the physical-fitness instructor marshalled his teenagers through the last course of the day .