baby, infancy
孩 centers on the earliest stage of human life: a baby or infancy. The meanings are tightly linked to the concept of a very young child.
孩 combines 子 (child) with 亥, which likely contributes the sound. The character expresses the idea of a very young child or infant.
A child 子 beside the character 亥, which looks a bit like a crib or a swaddled shape. Picture a baby resting in a crib: 孩 means baby or infancy.
For ガイ, imagine a baby saying 'guy' as its first word: guy -> ガイ, and the baby 孩 is just starting to talk.