What is the secret that Tim is supposed to keep at the
end of the chapter?

Ibelieve it is deiphobus

First when sexton says; "a thousand doors ago" in the poem "young" she uses a hyperbole, an extreme exaggeration, to explain what time in her life she is talking about. next sexton says; "the wise stars bedding over me" in the poem "young" she uses personification, gives human characteristics to  non-humans, to explain that she is outside at night, with the stars above her. third sexton says; "the boards on the house were smooth and white as wax"  in the poem "young" she uses  imagery, concrete or abstract images that  appeal  to the scenes, to show that the boards on the house were smooth. and she uses a simile when she says "white as wax" which means she used like or as to compare two subjects. in this line she compares the color white and wax to explain further what color the house was.