Girl and family flee destruction of earth and try to survive on alien world. Book is a diary

Girl and father and brother? leave earth on an ark ship headed to another planet. After several years? they arrive on new planet, but it is much harder to survive than anyone thought. I specifically remember crops not growing the way everyone thought they would. Wheat is crystalline and practically inedible, etc. They get through the first year and realize they should be recording their history someway. They find out the girl had been keeping a diary and begin reading it aloud. The first sentences are the first ones of the book.