The Railway Children - E. Nesbit

still more gaily. “I have grown already, haven't I, Mother?”
“Yes,” said Mother, “oh, yes,” and Bobbie and Peter felt her hands tighten on
their arms.
“Poor old Mammy, you ARE tired,” said Peter.
Bobbie said, “Come on, Phil; I'll race you to the gate.”
And she started the race, though she hated doing it. YOU know why Bobbie
did that. Mother only thought that Bobbie was tired of walking slowly. Even
Mothers, who love you better than anyone else ever will, don't always
understand.