aaifor,
One thing that you may want to consider for the Extension is that if it is denied, the 10-yr multiple entry visa gets automatically canceled (see USCIS website) and your parents will have to apply for a new visa the next time around, which might prove tricky.
I am in the same situation, My Dad got a 3-M stamp and Mom got a 6-M stamp. They have been visiting US for the past 14 yrs and this is 1st time this has happened. They have never extended their stay during their past visits.
I have an appt w/immigration officer in SF on Monday (12/15) to see if I can get my Dad's I-94 re-stamped for 6-M. If it does not pan out, my parents will go back before my Dad's I-94 expires. Applying for extension is not a wise option, in my opinion, since it runs the risk of getting the 10-yr visa canceled, which is a bigger headache than just going back early.
I will convey my experience after the appt on Monday. If successful, I will write about the process I followed.