Childhood in this novel is presented as something that is truly magical and wonderful, but ultimately, something that is evanescent, ephemeral and fading. This is the overwhelming message of the novel as Tom slowly begins to work out that as he goes back in time he is seeing the inevitable passing of time occur. This is reflected starkly in the comparison between the massive garden behind the house that is the site of so many adventures for Hatty and Tom and the miserable back yard that the garden has become in the present. In the conversation that Tom has with Mrs. Bartholemew, Tom is forced to confront the fact that childhood is a stage that inevitably passes. Note what Mrs. Bartholemew, the adult Hatty, says to Tom, about what she learned after the tall fir-tree was struck with lighning and it fell:

And then I knew, Tom, that the garden was changing all the time, because nothing stands still, except in our memory.

Childhood is therefore one of those things that inevitably passes and does not stand still. It is only resurrected through Mrs. Bartholemew's memories. Tom, at the end of the book, has learnt that childhood is something that will inevitably pass, but that the magic and wonder of it is something that can stay with him into his adult life if he chooses. This is something that is symbolised at the end of this novel when he hugs Mrs. Bartholemew as if she were a little girl.

The original question had to be edited down. For Pierce, childhood is something that is passing.  It is shown to be temporal.  It is not permanent.  For Tom, the discovery of the garden is something that happens immediately, without any premeditation.  At first, he is unaware of the fact that the garden is passing him by in terms of Hattie’s aging.  The garden is slipping through his grasp, and he is unaware of this passage of time.  In many respects, Pierce shows this as childhood, itself.  It is reflective of the stage of life where there is innocence is evident.  Innocence is fleeting, as the garden itself is.  The passage of time and the passage of the garden are reflective of how childhood is a stage on its way to adulthood.  As Tom recognizes the garden is gone, there is an immediate sense of regret of longing for that which is gone.  It is here where one sees another example of Pierce’s understanding of childhood as a part of the path to adulthood.  Childhood is something seen as a part of something larger.  It is a condition in which Tom interacts, unaware of its fleeting nature.  It is a source of great anguish when the reality hits that it has passed.  The only consolation is our memory of it, something that Tom recognizes in hugging Hattie.