Well I'm a bit prejudiced because I live there, but I'd recommend Huntington Beach. It's a nice wide sandy beach and you can watch surfing nearly any day. Easy to reach even on surface streets from DL - although it's probably easiest to go down Harbor to the 22 west to Beach and then straight down Beach Blvd. I go to DL all the time and it takes about 25 min in light traffic.
Newport ( straight down Harbor -- with a few changes near the end ) has a nice "Mother's Beach" area -- more protected, fewer waves for very little ones.