Well, for starters don't get to the airport too early. Most airline counters don't even open until 5am and that early, nothing at the airport will even be open. If the flight is at 6am, get to the airport at 5, no earlier. If it's a 25 minute drive, leave at 4:30. Wake the kids up at 4 and be ready to leave immediately. Pack them something to eat for breakfast (and lunch if this is a nonstop flight to CA) on the plane, otherwise feed them breakfast in the car. Do something fun like donuts and chocolate milk. You can pickup the donuts the day before.
You don't need to do anything to prep them for this ahead of time. YOU are the one who needs to prep and make sure everything will be ready to go in the morning, so minimize your morning routine as much as possible: take out all trash the night before, set the thermostats to turn off or go on vacation mode at a set time, load the car with all the luggage the night before and leave it in the garage except for one carryon where you can throw everyone's pajamas into in the morning and any last minute items you need to use in the morning. Have the kids pack backpacks and such the night before. Make sure any electronics are fully charged and turned OFF and put them along with charging cables in the backpacks in the car. Don't let them play on the ride to the airport. Save it for the plane, or you may end up with half dead batteries before the flight.
Just get the kids to bed early and let them know when you wake them up it will still be dark out so they will be prepared.
And expect to all be EXHAUSTED the day you arrive in CA and for the kids to be ready for bed by like 4 or 5pm. That part is the worst thing about flying to the west coast with little kids.