It depends how far you want to venture out. Its crowded in Southern California so you are going to hit a lot of traffic no matter where you go leading up to and after a fireworks show. We usually stay home because we can see about 3 different huge firework shows from our backyard. DLR gets extremely busy and there are many people who come just to watch the fireworks show. Now in the past they used to combine the current fireworks show with the old 4th of July one. I don't know about the recent past years because the current one is pretty long anyway. A lot of locals head over to the Mickey and Friends Parking structure and watch from the top floor and then leave. You can almost do anything during the day and the only places I do know are extremely busy are the theme parks and the beaches.
If you want to stay in the area, have you looked into the Discovery Science Center which is off the I5 in Santa Ana. You can't miss it, there's a big dinosaur and the Space Shuttle. I think that may be geared towards your 6 yr olds.
