All:
I have been using Hotelplanner.com to book our group rooms for years - I have a great GE discount for work purposes, but Hotelplanner usually kills the rate for groups of 11 or more. All you do is put in your city (or even field address), the distance you are willing to drive from hotel, and the rate you are willing to pay - the site sends the request to all the hotels in the area and they "bid" on your group - Doesn't cost you anything for site and your parents can pay individually - works well for us.