There's Adventure Aquarium and the Children's Garden [carousel, mini-train, butterfly garden and other stuff] in Camden, then take the Riverlink Ferry across the Delaware to Penn's Landing where you can tour a submarine, look at the outside of the tall ship Moshulu [it's currently a restaurant, no idea if your kid would like it], rent a kayak or paddleboat (the ones you row with your feet), and check out Spruce Street Harbor Park.

You should probably also spend a day at the shore, visit one of the boardwalks, get soft pretzels and water ice and boardwalk fries and a hoagie. There's the beach, but there's places you can rent boogieboards and skim boards if you want (not available everywhere, so check before you decide which beach to visit).