I cannot tell whether Diamond knew what she was thinking, but I think I know.

She had heard something at church the day before, which came back upon her--something like this, that she hadn't to eat for tomorrow as well as for to-day; and that what was not wanted couldn't be missed.
So, instead of saying anything more, she stretched out her hand for the basket, and she and Diamond had their dinner.
And Diamond did enjoy it.

For the drive and the fresh air had made him quite hungry; and he did not, like his mother, trouble himself about what they should dine off that day week.

The fact was he had lived so long without any food at all at the back of the north wind, that he knew quite well that food was not essential to existence; that in fact, under certain circumstances, people could live without it well enough.
His mother did not speak much during their dinner.

After it was over she helped him to walk about a little, but he was not able for much and soon got tired.