We have stayed at the JW in Santa Monica, which is a great location on the beach(you might have to cross a street, but it is no problem), and you can EASILY walk to the pier with the carnival rides, and restaurants, and downtown. It is by the boardwalk, and you can rent bicycles and ride for miles! Unfortunately, we are always a bit miffed with Marriott when it comes to Southern CA(esp.)San Diego, because there aren't many great hotels right on the main beaches. But, this JW is in a great location, and if you can stay here, you and your kids will love it!
Also, if you go a bit further south, Newport Coast Villas are a beautiful choice. They are timeshares, but you can book them trough Marriott by the night, too, if there is availability. They are all 2 bedrooms with a full kitchen, washer and dryer, lots of pools, fun things for the kids, and great views of the ocean facing west, so the sunsets are fantastic. They are up on the hill overlooking Crystal Cove Beach, which you can walk to, but the Marriott has a shuttle, too, if you don't want to make the hike up. You can easily drive to Newport Beach, Balboa Peninsula, and Laguna Beach, too.