Bikash Kc and Istamitra is right. You do not need to file two seperate application, (You may include your spouse and your unmarried children under age 21 years as co-applicants in your application for the same extension or change of status, if you are all now in the same status or they are all in derivative status.), hence only $300.00 will suffice for your application. As far as reason goes, you should have believable reason why you want to extend their stay. You have to let them know and make them believe that whatever reason you need that extension for could not be done while they were here in past six months. Simply family reason, or medical reason or need to see more places will not serve the purpose.

My suggestions is have one of them go to the doctor and make them write you a report saying that he/she needs further medical investigation/treatment, or something else that would justify six more months of stay. One more thing, make sure you apply your extension before it expires and you can apply very close to the expiration day. Normally it takes two to three months for USCIS to answer you  for acceptance or denial. Even if it is denied they will give you 30 days to leave the country. As long as you apply before the expiration of your visa and leave the country wihin 30 days of receiving that letter, YOU WILL NOT BE OUT OF STATUS. That way you can buy few months, just a suugestion.

For further info visit USCIS website http://www.uscis.gov/files/form/i-539instr.pdf