First of all it is important to know what a thesis statement is. A thesis statement usually appears at the middle or end of the introductory paragraph of a paper, and it offers a concise summary of the main point or claim of the essay, research paper, etc. It is usually expressed in one sentence, and the statement may be reiterated elsewhere. Now, the one child policy is a policy that was implemented in China to control the population growth. They did this by limiting the amount of children that a family could have to one child per household. So, a thesis statement about the one child policy would be a one or two sentence overview and summary of what it was all about. I hope my answer is of some use too you :)
