They would only have to borrow more money from China to do it.

Nope, the gov has gotten smarter than that. It's called quantitative easing.... essentially creating money out of thin air.
It is too expensive to borrow from China.
This is what my econ professor once said:
US borrows $100 from China, and pays 5% interest rate.
US use that $100 to buy $100 worth of raw materials from China.
US then use that $100 worth of materials to print $100 bills. It costs $0.02 to print one $100 bill.
The US then repays China 5% interest, using the $100 bill created using $0.02 of materials.
The cost for US to borrow $100 from china at 5% rate is thus calculated as $0.02 x 5% = $0.001.
And how does the US pay that $0.001? You guessed it, the US borrow another $100 from China, to buy $100 worth of materials, and to print $100 bills that cost $0.02.
There, he said that is why US cannot go bankrupt, as long as USD is the world reserve currency.