As with most things, it really depends on what you expect to do when you "go to the beach".
If you expect a place where you can sit on the beach and swim in the surf with your kids, that is one set of options. As a PP mentioned, Corona Del Mar is nice. If your kids are smaller or not at all familiar with the beach, they might enjoy Mother's Beach in Long Beach more as you are really at a lagoon. You can still walk down to the ocean.
If you prefer the hustle and bustle of a busy beach with shops, dining and a pier, then you'll have to make sure your choice is near one of the several piers along the Southern California coast. Newport Beach is an excellent choice, but so it Huntington Beach. Both are busy, offer bike rentals (I know Huntington has a 4-person "bike" that is really more like a pedal driven cart ... not sure about Newport) and have dining and shopping within the immediate area.
If you would prefer to slow down a little and enjoy some small town charm, I prefer Seal Beach. It is less popular and a bit older, but still very nice.
If you like the idea of taking your pet down to the water then Huntington has a "dog beach" complete with a souvenier stand. It is popular with the locals. You might enjoy just sitting and watching all the dogs play in the water, but you don't see too many people making that stretch of beach their family play area.
Not all beaches have fire pits, but for those that do, they are very popular. If you go to a beach near a fire pit, KEEP YOUR KIDS AWAY ... AT ALL TIMES. People often put out their fires by throwing sand over them. This doesn't put out the fire and instead creates an oven. Kids have accidentally fallen in or stepped in only to burned, many hours after that fire was supposedly put out.
I know it probably sounds a bit complicated, but to get the most out of your beach experience, it is really a good idea to know what you expect first. Each of the beach areas has something a little different to offer.