孩 is a bound morpheme (b.f.) in Mandarin — it cannot stand alone as a word. It always appears as part of a compound noun referring to a child or young person, such as 孩子 (child), 男孩 (boy), 女孩 (girl), and 小孩 (kid). You will never use 孩 by itself; it must be combined with another character to form a complete word.