I've talked about the Aquarium of the Pacific in Long Beach on this blog before. And I've mentioned that, for the money, I'm not that overly impressed, especially from an educational stand point.
However the Cabrillo Marine Aquarium checked off several of my boxes, including price (it's suggested donation for a family of 4 is $12, the price of one child ticket at AotP), information (there were docents walking around answering questions) and education (there's a aquatic nursery and exploration center with several specimens under microscopes for kids of all ages to check out).
While CMA is not quite as glitzy as AotP, there's no movie theater, cafe, or some guy taking your souvenir picture when you walk in the door, it does, in some ways, come off as more child friendly. The Exploration Center in particular, where the kids can step "inside" an aquarium and look at you, looking at the fish. There's marine themed puzzles, books, all sort of jars and bones to pick up. There are costumes to try on, a scuba diver, a turtle and watch yourself act on tv. There's even a marine themed puppet show for the kids to play with.
If you're planning a trip to San Pedro to check out this working aquarium, pack a picnic lunch too. The parking lot that serves the aquarium also serves a beach, where loads of families were spending some fun in the sun time. The parking fee is $ 9 for the day or $1/hr. cash only and there's no ATM at the aquarium.