Kendra continued scouring the tiny house, investigating every piece of furniture, searching every closet, checking behind every miniature painting on the walls.”
-- Narrator (Collecting Clues )

Importance: Kendra is determined to find out what the keys fit. This shows her level of determination. That determination proves to Grandma and Grandpa that Kendra and Seth could be the right people to take over the operation of the Preserve some day.

That seems like something a witch would do.”
-- Seth (The Ivy Shack)

Importance: Seth has just found Muriel for the first time and he is immediately suspicious of her actions. The fact that he makes the leap to ask if she is a witch makes it clear that he is open to supernatural explanations, which makes him later open to the presence of all the magical creatures on the Preserve.

Drink the milk.”
-- Narrator (Journal of Secrets )

Importance: These are the only words written in the Journal of Secrets once Kendra manages to get the locks...