Hello Cassandra, thank you for taking the time visit the Disney Parks Moms Panel.
At this time, a five-day pass is the maximum number of days you can purchase. There are no additional discounts beyond a 5-day pass. I spoke with a cast member at Ticketing and was told a 6-day visit would require the purchase of a 5-day pass plus a non-discounted single-day pass.
Despite having one less day than you have originally planned, Cassandra, you will likely find that there is ample time to explore both parks with a five-day pass. Your mid-January trip to the Disneyland Resort is typically not a busy time, with the exception of Martin Luther King Jr. weekend. I have visited the parks during extremely busy times such as Easter and peak summer months, and found that a 3-day trip was a good amount of time for our family of four to experience the parks.
I hope this information helps with planning your upcoming trip. Please swing by anytime if you have further questions.
Have a magical time at the Happiest Place on Earth!
-Kiki