Hi there Foster,
Welcome to planDisney and thank you so much for your question! I'm thrilled to hear that you'll be taking a big family trip to the Happiest Place on Earth later this fall. I took a trip with some of my extended family earlier this year and we had so much fun!
I'm happy to share that you all are able to utilize the entrance into Disney California Adventure Park accessible through 
Disney's Grand Californian Hotel & Spa. Additionally, since you all are staying in one of the 
hotels of the Disneyland Resort, you're all also eligible for 
Early Entry each morning of your stay.
With this fun perk, you'll be able to enter one of the parks 30 minutes before they open. You'll be able to enjoy select attractions and typically there are also 
Disney PhotoPass photographers available as well for some of the best lighting of the day for a group picture!
You can check the 
Hours & Events Calendar as you get closer to your trip to see which park is available for Early Entry each morning and make sure that you have your 
theme park reservation set to it. The calendar is available about 6 weeks out, so you'll want to check it in mid-July or so.
Thank you again for your question, Foster! I hope you have a wonderful trip to the Disneyland Resort with your family.
Have a Magical Day,
Greg