I am David
The Silver Sword
A series by Morris Gleitzman which starts with "Once" followed by "Then", "Now" and "After".

These are easy books to read but the themes are complex and can be quite sad.
Really interesting selection on this Good Reads list.

http://www.goodreads.com/list/show/17000.Best_Children_s_Books_about_World_War_II_1939_1945_