In San Diego, you are now allowed to sit on the beach, lay on the beach, loaf on the beach, etc. Your are expected to keep your family group at least 6 ft away from others. Things can change by August and things can change with only a one day notice. So, I wouldn't base an entire vacation around going to beach, but if the beach is just one part of your vacation, then I believe you'll be able to visit the beach in August and it won't be a total loss if you cannot.
There were beaches that opened in certain parts of San Diego County before the all of them did. Each city can make their own rules and even if there are rules above them (like state mandated rules), it's the city that you are in that will enforce the rules or not. For example, I live in a city in San Diego County and most people do not wear face masks in public, even though it's a state mandated rule. My particular city just doesn't care and won't enforce anything unless someone complains above their head.
Beaches I recommend close to Disneyland - my favorite is Crystal Cove State Beach which is in Orange County. If you do want to drive down to San Diego, I recommend La Jolla, Oceanside, Del Mar and Torrey Pines.