If you cannot make out this channel, then maybe the surf is too high.
The following comes from the DiveBums site.
With all the issues around using Children's Pool, people sometimes use South Casa instead—the other possible entrance to the wonderful shallow, rock reefs off just outside the seawall is at South Casa. South Casa is a much less easy and less safe entry/exit than is the Children's Pool, but is possible if conditions are good and you know what you're doing. To get down to the beach at South Casa, use the stairs just south of the lifeguard tower.
Before you descend to the beach, however, look at the water in the small cove below you. If the water is calm enough to see the rock structure in shallow water, you will discern a sandy channel towards the left side of the cove, running parallel to the shore on the left side. If you cannot make out this channel, then maybe the surf is too high for you to use this entrance and you should go somewhere else.
There is a rip which runs in this sand channel, making the entrance easy and the exit not-so-easy if there's any surf. Go out through the sand channel and turn right. Descend wherever you want. You can dive in the cove on the south side of the seawall, or you can go around in front of the seawall and do the more traditional Casa Cove dives.
In front of the seawall are a series of rock reefs, in long parallel bands, running nearly parallel to shore. You won't get into water deeper than about 30' unless you go far offshore. On your return, try to go back through the sandy channel, against the rip. If there's much surf and much of a rip going, then go somewhere else next time it looks like this!
If you can't get back into the channel—the surf's gotten too big and you're worried about all of the submerged rocks in the shallow water or you can't handle the rip, then you can exit on Wipeout Beach.
Wipeout Beach is a shore break. That means that there will be nasty, big waves breaking right on the beach. Don't take your fins off. Don't take your mask off. Know where your regulator is. And crawl out.