She used to teach them when they were children.
它是表示时间，还是作为them的定语？
She used to teach them when they were children.
when they were children 是时间状语从句，修饰 used to teach，说明“曾教他们发生的时间”。它不可能修饰“人”。
She used to teach them when they were children.
【翻译】当他们还是孩子的时候，她曾教过他们。