It doesn't seem that most Americans know we fought with the Chinese. I consider myself to be pretty cognizant and had no idea until I was late into my BA in history.
This has more to do with the way history is taught. Due to the nature of standardized testing, history is presented as a series of dates and facts that is unchanging. Anyone who studies history beyond high school will understand that history is really the study and understanding of the past through ideas and viewpoints (historiography).
There is also the problem of cramming 400 years of U.S. History into a year, for comparison during graduate school I took an entire seminar that focused on three days leading up the Russian Revolution. Some class periods we analyzed events by minutes.
High school history also has to be comprehendible by a large diverse group of students which limits the ability for class analysis. I did really begin to understand the reality of history until my sophomore year of undergraduate.
As to your original question "Was there a decision". It is highly doubtful that a group of powerful educators decided to begin a pro-china high school history program. The more reasonable explanation is that most high school classes barely get past WWII, so the idea that we fought the chinese is not that important.
The Korean War is often called "the forgotten war". It gets forgotten between the more obvious stories of WW2 (Nazis!) and Vietnam (protests and stuff). So it's not anything about it being covered up, it's " you're lucky if Americans remember that it existed"