What beaches in Southern California allow you to dig holes in the sand for bonfires?

NONE. If a beach allows Bon Fires they require them to be in an existing Concrete Fire Ring. So if you don't get to the beach in time to secure a ring..you are out of luck. So yes there are days that to have a good chance of getting a pit you need to be there early in the morning and have someone there who can watch it all day. In LA there is Dockweiler. In Orange County you have Huntington State Beach, Bolsa Chica State Beach, Corona Del Mar as well as a few others a bit farther south.

None that I know of. But Corona Del Mar is a good beach with firepits. if you get there late just ask an older couple or a group with young kids if you can use theirs once they leave. Since they tend to leave early :)

"Can I bring a barbecue or make a bonfire on a Los Angeles County Beach? No barbecues or bonfires are allowed on any of the Los Angeles County Beaches (or parking lots) except in the fire pits at Dockweiler in Playa del Rey. Cooking fires are allowed in the barbecue grills at Burton W. Chace Park in Marina del Rey. (County Code section 17.12.370)" Try Ventura County.

None. Dockweiler Beach is one of the very, very few beaches that have fire rings. You can build a fire in one of the fire rings, but you absolutely cannot build a bonfire on the beach. http://www.beachcalifornia.com/beach-bonfires-california.html