Statement:
The term “Santo Niño” is for sure not biblical at all. Rather, it was just only invented by the Catholic priests to promote that devotion they invented.
Well, they were mistaken on that point of view because the term “Santo Niño” is a Spanish term for “Holy Child”. Both of those terms are biblical.
Acts 4:27-28 KJV Bible
“For of a truth against thy HOLY CHILD JESUS, whom thou hast anointed, both Herod, and Pontius Pilate, with the Gentiles, and the people of Israel, were gathered together, for to do whatsoever thy hand and thy counsel determined before to be done.
Luke 1:35 Spanish Bible
“Respondiendo el ángel, le dijo: El Espíritu Santo vendrá sobre ti, y el poder del Altísimo te cubrirá con su sombra; por eso el SANTO NIñO que nacerá[a] será llamado Hijo de Dios.”
Therefore, it is Biblical since it can be found in the Bible. Remember that there are different languages and terminologies used in the Bible, but it has the same substance or subject matter in which that refers to the childhood of our Lord Jesus Christ.