* Peasants (a crude/outdated name, but its the one commonly used) have little to no government punishment for having more than one child. Most don't because they cannot afford it and do want to attempt to give their one child the best they can to enable them to move up in society.
* Farmers (a subset of peasants) also can't afford it, but having more hands to help many times outweighs the costs of education and health care which they can't afford for even one child.
* Couples of any class who are single children are allowed to have 2 children.
* Wealthy that rely on government for their continued prosperity (which is most) usually do not have more than one child despite their ability to afford government fines and lack of social support.
People with enough money or connections can have all the children they desire. Men who divorce and remarry get at least one child per wife, and women who can pay fines can have more than one child also.
Really, given that population trends in most countries are headed towards or are under replacement level, they could probably just ditch the n-child policy altogether, but I don't think the government of China is capable of making such a momentous change any time soon.