If you like fast and gnarly DH you will love mt Wilson. It's mostly a shuttle spot. The drive from Santa Monica is about 20 minutes without traffic and an hour with. There's lot of back country trails that are worthwhile, but most guys just do the 15 mile downhill run from the top. It takes about 2 hours and you go through several different types of terrain (loose and steep to fast and loamy). This is where you book your ride
https://www.scoaoutdoors.net. There's also some less techy and beautiful trails in the area like chantry flats and dark side.
Like someone else said Simi Valley (45 min drive) has some great technical riding. But there's no lift.
Snow summit (2-3 hr drive) is really the only lift access park around, it's worth a visit, but it's pretty small and doesn't have much tech.
Snow Valley (2 hr drive) is a lift serviced mountain, but all of their trails are just old school back country with no purpose built features. They recently started running the lifts for bikes in the summer. I haven't been since they opened it to bikes, but I hear they have a ways to go.
Sky Park ya Santa's Village (2hr drive) is a self shuttle bike park. Meaning you pedal to the top, it's a mellow pedal and totally worth it. They have several very fun trails and lots of good Mtb culture. They also have a bunch of great restaurants and non-biking things to do. Totally worth checking out and bringing the family.
This is another shuttle service that runs in the greater LA and OC areas:
https://www.crazybearbikes.com/shuttle/
There's other good shuttle spots, but you really need a crew with trucks for them.