a young child: a baby or small child, from a newborn through the early years
This is the ordinary Greek word for a child who is still little, anywhere from a newborn baby to a child of a few years old. It is a tender, everyday word, the kind of word a parent or neighbor would use, and the Gospels reach for it constantly: for the infant Jesus, for John the Baptist as a baby, for the little girl Jesus raised, and for the small children people kept bringing to him.
Because the picture it paints is so vivid, the same word is also used in two natural extensions. Jesus and the New Testament writers point to a young child as a living illustration: to enter God's kingdom you must receive it the way a small child receives a gift, with open and trusting dependence. And because the word is warm, it became a fond way to address people one loves, the way an older person might call those in his care his dear children.
All of these stay anchored to the same plain picture: someone who is still young and small. The shifts are in how the word is used, not in what it means.