In summary
(1) your parents does not need to PAY TAX on their Chinese pension, according to Sino-US tax treaty
(2) Your parents need to file FBAR and FAICA forms to REPORT foreign asset, not pay tax.
http://www.irs.gov/Businesses/Comparison-of-Form-8938-and-FBAR-Requirements
(2.1) 8938 form, 国外财产超过5万美元(包括房产) 以后卖房赚钱要缴税
(2.2) FBAR reporting 国外银行账户超过1万美元 以后存款股票赚了钱要缴税。