The Mystery of the Disappearing Cat by Enid Blyton
"You may all have a picnic in the garden," she told Pip. "Daisy, see that everyone behaves, please, and if you don't have enough to eat, go and ask Cook politely - politely, remember - for some more bread-and-butter."
"Yes, Mrs. Hilton. Thank you very much," said Daisy. The children watched Pip's mother going down the drive at half-past three that afternoon, looking very smart. They were glad that they did not have to dress up and go out to tea. It was much more fun to have a picnic tea and wear old shorts and shirts!
They had a lovely tea, and went in twice to ask Cook for some more bread-and-butter. Daisy went, and remembered to ask very politely. There were ripe plums and greengages as well to eat, so it was a good tea.