Traffic is likely to be really bad after about 2 or 230 heading south on the 5 near downtown LA and south. If you can hit the Newhall Pass by 1PM, then you'll hit some traffic but not as bad as if you hit it after 2 PM.
Check out
Sigalert.com - Personalized Traffic Reports around 1, 2, and 3 PM to see what it is like.
If you can leave at 4AM, and stop once or twice for 15-20 minutes you probably won't do much better.
When we travel with our kids we are loaded with two laptops with tons of movies, sweets, books, and MP3 players.
Another thing we do is use the McDonalds Trip Planner which shows where all the McDonalds with Playlands and Playplaces are along the route. Write those addresses down and when the kids start going nuts, you can plug those into your GPS and pull over and let them de-energize.