What is the literary term for this technique of hinting at things to come, such as when Aaron smiles up at Hannah in The Devil's Arithmetic?
The literary term you're referring to is called foreshadowing. Foreshadowing occurs when an author hints at events that are to come. Authors use this technique to build suspense. Think of them like breadcrumbs that you, as the reader, follow in order to find the story's treasure.
In The Devil's Arithmetic, there are several examples of foreshadowing. The example you correctly identified happens during the Passover Seder, during the reading of the four questions. Hannah encourages her brother and she is the only one who knows what is going to happen.
Another example of foreshadowing is when Hannah says she does not want to attend the Passover Seder because she is tired of remembering. This foreshadows the event of her traveling back in time.
Another example of foreshadowing is when Fayge makes a reference to the Angel of Death. This foreshadows the Nazi concentration camps and the deaths that took place there.
Aaron’s smile is an example of foreshadowing, when an author hints at the future.
During the reading of the four questions, Hannah’s little brother Aaron is nervous. He thinks he is going to forget something. Hannah remembers how she used to be nervous the first time she did it, and she tries to distract him. During the remembrance, she gives him encouragement. When a page gets flipped, she helps him.
Hannah reached out and smoothed it back for him and he smiled up at her gratefully. He has the greatest smile, Hannah thought… (ch 2, p. 14)
This incident demonstrates that Hannah is caring and forgiving. She worries about her brother, and has empathy for him. His fear and her reaction foreshadows the coming events, because Hannah is the only one who knows what is going to happen in the Holocaust during the book.