Every book, poem, short story, play, song lyric, etc. has a theme. The theme is simply what the book is about, and there can be more than one theme.
Obviously, one of the themes of "A Death in the Family" is that someone in a family has died; you expect that just from reading the title. You also expect that a theme might be grief or loss or family relationships or the like. The eNotes section on themes has a good discussion on several of the themes of this book. I've pasted a link to that page in the Sources section below. I've also given you links to other sites where you can find more information.
I hope this helps. Thanks for using eNotes!