The distance to the beaches from DL remain somewhat constant over the year....
Huntington, Newport, Balboa Peninsula are all close for day trips.
If you want to stay overnight (I think that's how I read your post), then:
Not sure where you're from, but up (and down) the coast there are plenty of beach towns that will be more fun for families than the urban ones in the LA area.
It should be warmer the farther south you go. San Diego is a whole other destination city, with zoo and animal park, Legoland, Sea World, an old rickety roller coaster (the best kind) overlooking the ocean, etc.
Further north, all the way to, say, Pismo Beach (200 miles?), you'll find more remote beach areas, and it should be colder.
If you're at the beach September 18 or 19, moon will be full or at perigee, respectively. High tides, so plenty of surf-watching, if you're into that kind of thing.
The ocean water is cold everywhere. No getting away from it, unless you go to Alcapulco or Hawaii.
"Here You Leave the World of California Today and Enter the World of, um, er, California Today."
Balboa is nice, but my favorite local beach is Corona del Mar. Unfortunately there are no hotels on that beach, just vacation rentals. However if you stay in the Balboa/Newport area its a reasonable drive to Corona del Mar which is less crowded as a general rule. Also there are fire pits for bonfires and tide pools to explore.