What do the children come to realize about Mr. Beiderman in the novel, The Vanderbeekers of 141st Street?

As the novel progresses, the children realize that Mr. Beiderman's hatred of the holidays comes from his tragic past of losing his wife and daughter in a car accident six years ago. When he reveals to the children that Isa has reminded him of his late daughter because they were both avid violin players, the children understand why his sadness has manifested itself as anger and grouchiness.