I'd suggest the beach at 15th Street - the 15th Street Surf shop is a great locally owned store that rents bikes, boogie boards, etc. and it's located right on 15th between Fry's Market (mini mart with beach toys, etc.) and the Stuft Surfer (burgers, ice cream, breakfast.) The beach there is nice and clean, and it's between the Newport Pier (on 20th Street) and the Balboa Pier (that's where you'll find Ruby's - I was typing too fast in my earlier post - the Fun Zone and the ferry to Balboa Island.) You'll also see a public parking lot and rentals at the base of the Balboa Pier. We've always thought 15th street to the Balboa Pier is best for families. Our kids like the playground right on the beach around 13th street -it's for the elementary school, but entirely unfenced and everyone uses it during the summer. (And I loved playing there as a little kid, too!)
If you're looking for extremely calm water, there is a public beach on the bay side at 10th street.
For those traveling without kids, 20th Street (and up) has lots of bars, restaurants, etc. And either way, if you're around 20th Street, the Crab Cooker restaurant is great for everyone (as long as they like fish!) Just be sure to get there early before the dinner rush!
Hope you enjoy Newport as much as we do!