It seems that "topic sentence" is what someone would call a thesis if they are not familiar with the technical term "thesis" or they are introducing the idea to young children and don't want to confuse them by using a technical term .

As I'm familiar with the terms, (almost) every paragraph in a (typical) piece of expository prose (usually) starts with a topic sentence. Thus, there would be many topic sentences. In fact, it should be possible to get the gist of the whole piece simply by reading the first sentence of every paragraph. A "thesis statement" would appear somewhere near the beginning of the piece, and would be succinct statement of what the piece will show or present.