孩 = 子 (baby) + 亥 (sound-alike tag). No hidden pig, no poetry.
The left side 子 is a drawing of a baby with its arms out, and it carries the meaning 'child'. The right side 亥 is there purely for the sound — 亥 (hài) and 孩 (hái) come out almost identical. Meaning on the left, pronunciation on the right. That's the whole build.