"small;" (masc.) "boy," "child;" (fem.) "girl"
12th cent. Perhaps from Latin ciccus "nothing," earlier "something worthless," but originally "the thin membrane surrounding the grains of a pomegranate." Presumably borrowed from an unattested Ancient Greek word *κίκκος (kíkkos) "shell of a pomegranate," hypothesized by Beekes (2008) on the basis of the Latin word and possible Greek derivatives κίκκαβος (kíkkabos) "small coin in the Underworld," κικκάβι(ο)ν (kikkábi(o)n) "nothing," and κικαῖος (kikaîos), a word of obscure meaning.
Ultimately of unknown origin.
Also the origin of the surname Chico.
Asturian chico, Aragonese chicot, Catalan xic, French chiquet, Italian cica
The sense of "small" was first and then was extended to children. The change from c- to ch- may have been reinforced by Basque, cf. txiki "small," "few," from earlier tiki. Also note 18th cent. colloquialism chicho "small child (who has begun to speak)," from *cic(c)us via double-palatalization distortion found in children's speech (compare niño and ñoño).