After a day (maybe 2..) they will be fine.
Just get into the LA time zone as much as is possible, depending on your arrival time, obviously.
If you arrive in the morning, stay up, do something and nap in the afternoon if needed, then go to bed at normal time.
If you arrive in the afternoon, stay up and go to bed normal time.
If you arrive in the evening/night, go to bed and get up at normal time.
Thats what we did, and we didn't suffer jet lag. We only went for 7 days and we were totally fine. The flight was pleasant and LAX is really central so there isn't much travelling after you actually arrive. (provided you are staying on the westside, which I presume you would given that its where the beaches are!)
La is absolutely worth going to. Its amazing. The beaches are fab, Santa Monica is beautiful and Venice beach is very hippy, but very, very nice. You will have an amazing time.