Except for the fact that China's economic growth is almost entirely dependent upon America's debt. Without the trade, China would go back to a poor communist state and be extremely unstable (Just like the USSR breakup)
That's not how it works. It's true that the U.S. owes the most to China but it's hardly something unique to the U.S. to owe money to many countries. The global economic system consists of many countries investing (read: LENDING) in other countries. Canada does the same.
Have you ever bought (invested) in bonds? That's how it is with international investing. Bonds have a fixed payment date that is contractually created. Unless there are certain provisions in place China just can't say "oh we want our money back" they will get paid on the time indicated on the bond. Indeed, the debt to China continues to grow because China continues to be **willing and eager** to spend their own money and invest in the United States.
Also, if you are smart, you would never want to see an "economical crash" of the U.S. (and I'm not saying that you do, as I assume you are one of our friendly northern neighbors...) So much of Canadian stability and security relies on the well-being of the U.S....
...(...but in case you do secretly hope to see the U.S. fall into distress) or would you rather see us be taken over by China, our natural resources annexed and our government stripped away, our industrial capacity harnessed, and our army, navy, air force, coast guard and arsenal of nuclear weapons, all be at the command of a foreign power whose 6 largest provincial capital cities contain the same number of people as all of Canada....right at your doorstep?