It is clear that what has made this text such an enduring classic since its publication is the way that it appeals to both children and adults equally. The author is examining childhood as a golden, glorious time that unfortunately is shown to pass inevitably as the years go by. However, as the elderly Mrs. Bartholemew shows, although time cannot be stopped, the memories of childhood can always be resurrected and summoned back. The garden itself is a place that conjures up a sense of real excitement in the minds of the children who read this book, but in so doing, it also helps remind the adults of their own childhood and their own days playing in such places. Note how this is made clear in the following description of the garden which comes in Chapter Ten, when Hatty properly shows Tom the garden for the first time:
She showed him all her hiding places: a leafy crevice between a wall and a tree-trunk, where a small human body could just wedge itself; a hollowed-out centre to a box-bush, and a run leading to it--like the run made in the hedge by the meadow; a wigwam shelter made by the rearrangement of the bean sticks that Abel had left leaning against the side of the bean house...
The list of secret hiding places that continues in the text is meant to appeal to both adults and children, who are likewise inspired by the sense of adventure and mystery that all of these hiding places and secrets in the garden yield. Although this is an example of children's fiction, it is therefore equally reliant on conjuring up happy memories of childhood for its various adult readers.
The easiest approach to take in answering this question is to suggest that the depiction of the childhood in Pierce's work is meant to be understood by both children and adults. I think that this becomes part of the work's legacy as children's literature. It teaches lessons to children about openness, inclusion, and optimism that almost strikes a note of melancholy in adults. It is for this reason that it becomes a work of children's literature that operates in a realm for adults, as well. In developing something that features lessons for how life should be for children and something that demonstrates an ideal for adults in how life could be, the work operates on both levels for appreciation in the lives of children and adults. I think that this becomes something that enables it to be understood by both groups of readers. Children can read about Tom's understanding of childhood as a form of escape or a world in which childlike imagination reigns supreme. Adults can wistfully examine what is depicted in the hopes of seeing their own lives mirror something from what is presented or examine how they have changed in a life where their own gardens might have expired.