The US "owes" China a trillion USD because the Chinese, investing on a national scale, need a safe investment that can support trillions and trillions of yuans. And in that regard right now, the US is the only game in town.
China's GDP growth is high, but that's because they're starting from a GDP that's much lower than that of the US. As with any logarithmic curve, growth starts quickly and then becomes harder over time. China has a ways to go.
The number of cars sold and the number of children enrolled in school are indicators that China does indeed have a much larger population than the US. Almost all of the BRIC nations have large populations, but are still in the process of turning that into wild economic growth. We'll see.
Other than the missing context for the numbers you provide, I agree with your general sentiment. There is a bit of schadenfreude among the US media whenever China struggles economicallly. This is probably because China is the US's biggest economic and military rival now that the Soviet Union is gone. It's natural to cheer for your team and boo the other team. I wouldn't take too much offense.