There are no discounts for large families. The only discounts for frequent visitors would be available are those that would be allowed with an annual pass, sometimes e-mails or post cards are sent out which provide a code for a discount, these seem to be sent to different areas and only the person's whose name is on the e-mail or post card can use the code.
The annual pass discounts are available online through the Walt Disney website and I thinks they take reservations over the phone during certain hours of the day. You need to have an AP voucher or the pass itself to get the discount. At this time you have to book within 30 days of your departure date and right now they only run until mid April
