The sculpture represents the infant Saint John (also recognizable by the camel skin that falls over his right shoulder).
Literature: Alan Phipps Darr - Giorgio Bonsanti, Donatello and His Florentine Sculpture of the Early Renaissance, Arnoldo Mondadori ed., Milan, 1986, pp.234-237.
Giancarlo Gentilini, I Della Robbia, Giunti ed., Prato, 1998, pp. 159, 185, 373.