China is a member of the NNPT, that means as part of its obligations under the treaty, it reports the location and other required details of all civilian nuclear processing facilities and have IAEA safeguards in place at those facilities.
I do not know the exact rules and requirements, but I fully expect those IAEA safeguards to require the plants to be able to account for every single gram of nuclear material they enrich, otherwise those safeguards would be useless. So skimming some materials off the top for bombs is simply not going to work because the whole point of those IAEA safeguards is designed to prevent that exact thing.
In addition, I would also expect the IAEA to have access to those civilian nuclear facilities so they can inspect and monitor operations and ensure its safeguards are sufficient and are being implemented correctly.
Being an established nuclear power, there is absolutely nothing stopping China from building a dedicated military enrichment facility especially for bombs right next to the civilian stuff, maybe even in the same building if they want. But the question would then be, why in the world would China want to do that?
I think you mixed up between Non proliferation treaty and IAEA.
Non proliferation treaty is the one that China or any other country need to sign if she want to license nuclear technology from say Westinghouse or Framatome now AREVA.
IAEA is an organization that promote peaceful use of nuclear energy with headquarter in Wien Austria . IAEA has the expertise to verify the peaceful use of nuclear technology they are not part of United nation but can be use as resident expert by the UN. Now the non proliferation treaty forbid member to reprocessed spent fuel rod for nuclear bomb That is where the accounting come from. But at the same time there is ambiguity. Because the treaty allow enrichment of natural uranium for peaceful use . That is the crux of the recent standoff between UN and Iran because iran insist they only use their enrichment for fuel rod and not atom bomb but there is no way to verify it unless Iran allow IAEA to visit their facility
And you are right the treaty does not restrict the original signatory from processing or enriching uranium!. In oterh wod IAED cannot demand to have accounting of China uranium processing facility. But she can check the use of spent nuclear rod from nuclear power plant built using license from Westinghouse or AREVA
Five states are recognized by the Non-Proliferation Treaty as nuclear weapon states (NWS): China (signed 1992), France (1992), the Soviet Union (1968; obligations and rights now assumed by the Russian Federation), the United Kingdom (1968), and the United States (1968) (The United States, UK, and the Soviet Union – the World War II's “Big Three” — were the only states openly possessing such weapons among the original ratifiers of the treaty, which entered into force in 1970). These five nations are also the five permanent members of the United Nations Security Council.
These five NWS agree not to transfer "nuclear weapons or other nuclear explosive devices" and "not in any way to assist, encourage, or induce" a non-nuclear weapon state (NNWS) to acquire nuclear weapons (Article I). NNWS parties to the NPT agree not to "receive," "manufacture" or "acquire" nuclear weapons or to "seek or receive any assistance in the manufacture of nuclear weapons" (Article II). NNWS parties also agree to accept safeguards by the International Atomic Energy Agency (IAEA) to verify that they are not diverting nuclear energy from peaceful uses to nuclear weapons or other nuclear explosive devices (Article III).
Anyway there is interesting article about the storage, life extension of Chinese nuclear weapon. And life inside the bunker deep in the mountain