The child's father sits her down on the chair. 依句點解?

The child's father sits her down on the chair. 的意思是:
那小女孩的父親讓小女孩安坐於椅子裡。
sits her down: 意思是"讓她坐好"
a child's father 會變為"有一個小孩的父親"，而用the child's father 是指"那小孩的父親"。如果說話的人想特定指出一個人，便會用the child's father，反之，a child's father 只是說"有一個小孩的父親"。

"The" is used when you are referring specifically to something/somebody.

"A" or "An" is used when you are pointing at something general.

in this case, the child's father is referring to that one father specifically so "THE" is used instead of A/AN

The child's father sits her down on the chair.

那個小女孩的爸爸把她放在椅上坐。

唔係 a child因為a child就轉咗個意思咖啦。 a child=有個小孩