Your topic sentence should introduce your essay — and until we know what particular facet of China you are discussing, and what your point of view is, there is no way anyone can provide you with a topic sentence.

Start assembling information, do your research, and find an interesting aspect of China – either past or present (or future, as in the Beijing Olympics) and start from there. Your topic sentence will grow out of your work.

http://www.snapchina.net/

Check out its Picture of the Day. Pictures there might give you an idea.

http://geography.about.com/od/populationgeography/a/onechild.htm

http://en.wikipedia.org/wiki/One-child_policy

http://www.reproductiverights.org/ww_asia_1child.html

http://law.case.edu/student_life/journals/jil/Notes/Schmidt.pdf