One way to hold this word: it is kid with a small soft ending stuck on, the same ending that turns a dog into a doggie. The ending does two things at once. It shrinks the thing, and it warms the voice of the person saying it. So kiddie is a very young child seen . Put that same word in front of another noun and it shrinks that too: the pool, the seat, the menu all come back in small-child size.
可以想成：kiddie 是 kid 後面黏了一個小小的軟尾巴，跟 dog 變成 doggie 是同一個尾巴。這個尾巴同時做兩件事。它把東西變小，也把說話的人的語氣變軟。所以 kiddie 指的是一個很小的小孩，而且說的人帶著疼愛。把同一個字放到別的名詞前面，那個東西也跟著縮小一號：池子、座椅、菜單，全都變成小小孩用的尺寸。
From the word’s history
Wiktionary gives the formation plainly: kid plus the ending -ie.
Wiktionary 對這個字的說明很單純：kid 加上字尾 -ie。整個畫面裡的「小」和「親暱」，都是這個字尾帶進來的。
Wiktionary, "kiddie"
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