孩 splits cleanly down the middle: 孩 = 子 + 亥. The left half, 子, is the radical and the meaning: a swaddled infant with arms out, the same glyph that names a "son" or "child" on its own. The right half, 亥, is pure sound. 亥 is the twelfth of the earthly branches and carries no childhood sense; it sits on the right only because in early Chinese it rhymed with the word for a small child. This is the textbook pictophonetic: one side tells you what, one side tells you how it sounds.

The pairing is old. In the seal script the left-hand 子 keeps its rounded, bundled-infant shape, while the right-hand 亥 holds the branched form the earthly-branch cycle adopted. The Shuōwén Jiězì glosses 孩 with the cooing of a baby, the small open-mouthed laugh of an infant who cannot yet form words. That sensory root, a sound before speech, is why the character first attached to the very youngest.

From that infant cry the sense widened outward through age: from the babbling baby, to the toddler, to any child, and finally to "children" as a class of people. Modern Mandarin almost never lets 孩 stand alone; it leans on a partner, most often the noun-softening 子 in 孩子 (háizi). The bare glyph survives mostly inside compounds, the way a brick survives inside a wall.

Picture the glyph as a nursery split by a curtain. On the left, 子, stands the child itself: a swaddled baby, arms thrown wide, the exact bundled shape you see in the radical. On the right, 亥, hangs a wooden cradle-tag carved with a calendar branch, a marker with no meaning of its own except to ring out the sound hái when you tap it.

So every time you read 孩, you meet the baby on the left and tap the sound-tag on the right: 子 shows you a child, 亥 sings you hái. Hold that division and the compounds fall into place. Hang a 小 above the cradle for 小孩, a little one; stand a 男 or 女 at the door for 男孩 and 女孩; and let the soft 子 settle back in for 孩子, the warm everyday word a parent uses for their own.