So she just loved Mother more and never said a single word that could let Mother know how earnestly her little girl wondered what Mother was unhappy about.

This needs practice.

It is not so easy as you might think.
Whatever happened--and all sorts of nice, pleasant ordinary things happened--such as picnics, games, and buns for tea, Bobbie always had these thoughts at the back of her mind.

"Mother's unhappy.

Why?
I don't know.