I believe most people would agree that the end of Qing Dynasty to Cultural Revolution was an "abnormal" period. Many historic events occurred: Scramble for Concession, Occupation of Warlords in China, National Revolution, Japanese Invasion and Occupation, Nanking Massacre, WWII, National War bewteen GMT and CCD... in 1949 (And then the Great Leap Forward, and the Cultural Revolutions... which were also catastrophic).
I am not sure where is the reference of 1 family giving 5 children came from, but I doubted such "productive" reference could sustain in such a catastrophic era in the Chinese history.
At the eve of WWII, the population of China was about 517.57 Million people, the total death (both civilians and soldiers) was estimated between 10 Million to 20 Million, about 2% - 4% of the total population. Other countries for reference:
Germany population in 1939: 69.85 Million, total death: 6.63 Million to 8.68 Million, about 9.49% to 12.42% of whole population
The Soviet Union population in 1939: 168.52 Million, total death: 23.4 Million, about 13.88% of the popluation
The United States population in 1939: 131.03 Million, total death 0.42 Million, about 0.31% of the population
But the reference did not mention clearly how the people died in this period of time. Highly appreciated if there is further reference. Let's exchange perspective more! ^_^
Source: Wikipedia (第二次世界大戰各國傷亡統計 ) <http://zh.wikipedia.org/wiki/%E7%AC%AC%E4%BA%8C%E6%AC%A1%E4%B8%96%E7%95%8C%E5%A4%A7%E6%88%B0%E5%90%84%E5%9C%8B%E5%82%B7%E4%BA%A1%E7%B5%B1%E8%A8%88>