Lol now I feel a real need to convince you to still come to SoCal haha!
So, here's my beach background to help you understand why I'm not a fan of the beaches here... I went to college in Miami and subsequently stuck around a few years after graduation soaking up rays on those white sandy beaches and warm, turquoise waters. Then, I continued my education in the Florida panhandle (Destin, FL) where my family still lives. There they have sugar-sand beaches and clear, calm water that's 80+ degrees in the summer. Then, I moved to San Diego, where there is kelp (gorgeous for diving, but not lovely to sunbathe next to once it starts to rot in the sun lol). The ocean is freezing and murky. It's windy and there are bugs...just not my ideal beach-going experience because I've been so spoiled with my other experiences.
NOW, that said, the beaches here do have things that the other places don't have - like tide pools! You can still get the kiddos to enjoy the beach. Personally instead of Newport, I would bring them south to San Diego. I'd take them to La Jolla near Children's Pool so you can take them to tide pools to find crabs, anemones, and sea stars, and let them see the seals and sea lions sunbathing on the shore. The beach in Coronado is also pretty wide and flat..perfect for wading. I love living in SoCal. Like you said, it's awesome to be 15 minutes from both the beach and awesome hiking trails at the same time. I'm just not a big fan of the beaches so I choose not to go very often. I love kayaking and going out on boats to whale-watch, I just don't like swimming in it. There are plenty of people that do, I just find it cold. Kids, as you said remembering your own trips when you were younger, don't think of stuff like that as much lol! They might be perfectly fine with it!
I strongly feel that
Disneyland is more "awesome" than WDW, which is why I don't want you to completely write it off just because the beaches aren't the same as those on the east coast. There are SOOOOO many other things to do out here that I think you would hardly miss having more than one beach day.