She is a children's favourite teacher
不对 如果是这样 a就是修饰 children 的了
而 children 是复数
如果你是要说 "她是一个孩子们最喜欢的老师"
就是 She is one of the children's favourite teachers
如果是要说 "她是一个孩子的最喜欢的老师"
则是 She is a child's favourite teacher