Besides all the visa documents

family pictures - your son with you and mom/dad, your son with your parents-in-law, your parents-in-law with their family hopefully will help.

The following links contain info about B2 visa application. Good luck.

http://www.usembassy-china.org.cn/chengdu/cons/visa_types.html

http://beijing.usembassy.gov/

http://www.path2usa.com/Visitor.htm#lnk1


http://uscis.gov/graphics/formsfee/forms/index.htm