It seems highly doubtful that Wargrave could have comprehended poetry at the age of two and then remembered it decades later. We'll attribute this statement to an old man's vanity.
In her Autobiography Agatha Christie wrote:
Difficult to know what one's first memory is. I remember distinctly my third birthday. The sense of my own importance surges up in me. . . . There is a tea table and it is covered with cakes, with my birthday cake, all sugar icing and with candles in the middle of it. Three candles. And then the exciting occurrence--a tiny red spider, so small that I can hardly see it, runs across the white cloth. And my mother says: 'It's a lucky spider, Agatha, a lucky spider for your birthday. . . .'