What does the book's concluding paragraph suggest about allowing the "dream of Wonderland" to become part of one's imagination?
Alice's growth during the trial reflects her growing awareness that Wonderland is an illusion. She reaches full height when she tells the Queen that her antagonists are "nothing but a pack of cards!" She grows to full size when she realizes she has control over her illusion. Throughout the book, Alice tried to make sense of the situations she's encountered, but at the end, she realizes this can't be done since everything is incomprehensible. This message is meant for the readers of the book as much as it is for Alice. Many critics have written about the meanings and interpretations in the book, but Carroll is telling us there is no definitive interpretation to the book, just as there is no definitive interpretation for Alice.
The last scene where her sister makes the comment to which you refer above changes the tone of the story to a childhood fantasy. The scene by the riverbank ends the book just as it began the book, and Alice's sister turns the dream into a story that Alice will enjoy telling her children some day. Her trip through Wonderland has been her journey from an innocent girl into a young woman, her coming-of-age story.