We are going those exact dates (26 - 30), and I picked the dates based on TouringPlans historical crowd levels only for 2012-2016, not the prediction for 2017.
My summary: Expect the weekend to be busy with APs, but enjoy the lighter weekdays. We're going on the weekend to ensure Fireworks and Fantasmic - although it could be possible since Fantasmic is new, it will be running every day during this time, but historically it has not.
Most SoCal kids will be in school during the week. Certainly some will pull their kids out of school to go, but it will likely be a small percentage of APs.
One thing we noticed on our trip during this similar timeframe in 2015 is that the APs would pour in during the afterschool hours, likely for PTN and
Disneyland Forever 60th celebrations. So plan to get to the parks early on the weekdays. We did Toy Story 3 times in a row and California Screaming twice in a row on a weekday because they were walk-ons at park opening (when DCA did not have EMH that day).
EDIT: I see you said you plan to have one off day, and that should be fine. According to the historical crowds, the Thursday before Labor Day is still okay, Friday is when things start to ramp up.