I have always liked the idea of Rosh Hashanah & Yom Kippur. While I don't celebrate these holidays I have always been impressed by their message. Rosh Hashanah starts the "Days of Awe" where those who celebrate try to make amends for their behavior & look for forgiviness to any they have wronged. Yom Kippur is the day they ask to be absolved. It is a very moving time. I also think it is a wonderful message for all children, so I am going to suggest some books that might help children understand these beautiful holidays.
In no particular order:
On Rosh Hashanah and Yom Kippur by Cathy Goldberg Fishman
Sound the Shofar by Leslie Kimmelman
Happy Birthday World: a Rosh Hashanah Celebration by Latifa Berry Kropf
Sammy Spiders First Rosh Hashanah by Sylvia A Rouss
Apples & Honey: A Rosh Hashanah Lift-the-Flap by Joan Holub