The difference in price between a 6-day ticket and a 7-day ticket is $10.65 ($10 + 6.5% tax) whether it's for an adult or a child, so it should've been $10.65 x 4, which is the $42.60 Robo came up with.
ETA: I checked the WDW website, and the price for 6-day hoppers for 3 adults and 1 child (since one of your children is a Disney adult) is $1571.94. Changing to 7-day hoppers makes the total $1614.54, which is a difference of $42.60.