niña de los ojos

pupil; apple of someone's eye

“2019, Watchtower Bible and Tract Society, Traducción del Nuevo Mundo de las Santas Escrituras, Zacarías 2:8 - Zechariah 2:8” — For this is what Jehovah of armies says, who after being glorified has sent me to the nations that were plundering you: ‘Whoever touches you touches the pupil of my eye.'

From Latin where pupa (“pupil”) was used as an alternative to refer to a girl (therefore pupil of someone's eye could also be interpreted as girl of someone's eye). The expression was then fully changed in Spanish to niña (“girl”) de los ojos.