The scenario you have put forward is that the child was born in the PRC with a Chinese parent. If that is the case then the child cannot legally leave the country without registering to get a chinese birth certificate in order to get a Hukou Ben in order to get an ID card and finally a passport I.e. they would HAVE TO become a Chinese citizen. If you had read the post below you would see that I stated that the Chinese don't allow dual nationality (good to see you're keeping up). In fact most Chinese administrative bodies would say that the child in question should wait until they are 18 until they themselves are in a position to decide their own nationality before they allow them to relinquish their Chinese nationality in favor of the other parents nationality. As a result the the family is left with only a few options
1. Try and find a way to get around the whole wait until the kid is 18 nonsense to legally relinquish the Chinese nationality
2. Accept it and stay in china
3. Leave china while still a Chinese national and head for their other parents homeland (they can do this on a tourist visa in their chinese passport). Once there they can apply for citizenship of that country. This isn't usually a problem for most westerners as our countries allow dual nationality. The problem that might arise though is if the child then tries to re-enter China they could find themselves in a spot of trouble and a potential bureaucratic nightmare.

Regardless of what you've seen or heard you don't know what you're talking about! I'd suggest you contact the British Embassy in Beijing if you'd like to verify my comments.