Parents visiting son studying in Beijing, together with his two children. Documents needed ?

Our son Håkon Lindbäck is studying Chinese language at the University of Oslo, but is now staying in Beijing as a student at Beijing University on an exchange program. We, his father and mother want to visit him in Beijing for 13 days in September - October 2012. (dept. Sept 26 - return Oct 9.)
His two sons, aged 14 and 11, will go and return together with us .All of us have Oslo Norwegian passports, living in and around Oslo
Our son is renting a room/small flat in Beijing. His sons will most likely stay with wim, my wife and I will stay at a nearby hotel . What documents are required in addition to the visa form.
Best regards
Karl A Lindbäck