By car would be best, but there is traffic the whole way during rush hours and not a carpool lane for most of it.
An alternative is taking a commuter train. It will require a transfer from one train sytem to another and a short taxi ride on each end.
The trains called Metro in LA, and Coaster in SD have a common station at the end of thier lines. They both run every 30 minutes during the day and this transfer is quick.
Depending on group size the cost of the trains(cheap) and taxis(medium) can be more than a one day rental.
A few rental companies have locations around DL. I'm not sure if thier are tour buses to LL like there is to universal and Seaworld.
LL has deal for buy and adult ticket get a kids free in the lego club magazines.