As I understand the idea of "literary focus", your assignment must focus on one aspect of the literary work. So you would have to pick character, theme, setting, plot, symbolism -- something like that.
In this novel, the garden is a symbol of regeneration. The new growth of the garden regenerates each one of the children, in a different way. So, perhaps you could choose one of the characters and show how the garden's re-growth also causes re-growth in that character. Or, you could just focus on the garden itself and describe how it is an effective symbol.
Another approach would be to focus on the language that is used to describe the garden, its location, the flowers that are in it. There is some beautiful language in this novel that describes nature, and this might be something unique that you could write about, that others in your class may not think of.
This novel is often compared to a modern-day fairy tale. Can you think of some fairy tales in which the characters are transformed as in this novel? If so, you could write your assignment on the theme. Beauty and the Beast comes to my mind. Also, in this fairy tale, a rose is an important symbol, just as the roses are in The Secret Garden. You could write about the rose.
Hopefully that gives you some ideas.