Growing up my mom would take us to Newport Beach and Balboa Island, so it's probably best fit for families. I'm not sure if I've ever been to Laguna Beach, and to me it's quite a bit further. Newport is the closest and easiest to get to from Anaheim.
Nowadays I find myself at Huntington more. The reason it's popular is because of the great waves and abundance of fire pits. If you aren't going to surf or have a bonfire, you'd be better off somewhere else. Everything else is very nice, but those are the major draws.
Seal Beach is cute but much smaller. Huntington and Newport have more tourist things available. There is a dog beach at Seal Beach and in Long Beach, as well as a mother's beach at Long Beach. In general, unless you are staying in Long Beach though, I'd say to avoid it. Or if you are visiting the aquarium, which is VERY nice.
In Newport Beach there is Newport Dunes, a beach with small lapping waves and a playground (I remember loving this as a kid). It's probably the safest as far as currents for the kids to swim in, but I know the last time I went there were jellyfish. That was many years ago, so I'd look into it. The nice thing about that too is it's a RV park so there is a liquor store right there if you want snacks. I think there were bathrooms with actual plumbing too.