I'm afraid you haven't thought that entirely through yet. CURRENTLY, China's economy depends upon endless American consumption, so tanking the USA's economy by demanding payment would be as bad for China as it would for the USA.
It's kind of like how Donald Trump works -- if you borrow $10k from a bank, and you can't pay it back, you're in trouble. but if you're like Trump and you borrow $100 million from a bank and can't pay it back, the bank is in trouble, so the bank will continue to lend you more and more until you're out of trouble. And allow you to pay it back over decades. Otherwise the bank itself becomes insolvent.
Anyhow; let's assume that China no longer needs a healthy US economy -- they have a large enough middle class that they can afford to consume their own crap, and become a self-sustaining economy no longer dependent upon world trade, like the USA was in the 50's/60's.
So, China demands repayment, even if it destroys the US economy. The US still has a few options, because they are a nuclear power, which can even involve wiping out their debt by wiping out the creditor -- essentially starting world war 3 in order to get out of debt.
But there are other options: For example, when the Chinese middle class reaches the 500 million mark, China may be too expensive to afford itself and will seek to export/offshore manufacturing by that time. Ironically, the USA may be affordable by then, with a large, well-educated, working class in desperate need of jobs.
The Chinese will own the factories, but the stuff will get built in the USA. Which, also ironically, will boost the US's economy and help the USA pay back China. Slowly, over decades, like Trump.