Philadelphia has fantastic activities and sights for tots. Best of all, these stops appeal to adults as well.
There's plenty of interest for the whole family in the Historic Area. Start at Independence Hall and be sure to hit the Liberty Bell and Franklin Court. Kids love the evening Lights of Liberty show. The City Tavern is fun for dinner and has a good kids' menu.
Head over to Penn's Landing to check out the Independence Seaport Museum before taking the ferry across the river to the Camden Waterfront to explore the Adventure Aquarium and the Camden Children's Garden. Back in Philadelphia, head to Chinatown for dinner. Make sure to look into discount passes—the RiverPass, for example, includes round-trip ferry travel and admission to the aquarium and children's garden. CityPass covers the seaport and ships, the aquarium and children's garden, as well as several other sights around Philadelphia, such as the Philadelphia Zoo and the Philadelphia Museum of Art.
Take a trip to Fairmount Park to watch crew teams row on the Schuylkill River. Don't miss the giant indoor slide at the Smith Memorial Playground and Playhouse—by the reservoir in the East Park. While you're here, you might as well check out the Philadelphia Zoo. If the weather's bad, make Day 3 a museum day. The Philadelphia Museum of Art, the grand dame, is always an option, but the Pennsylvania Academy of Fine Arts is equally interesting to kids and has a wonderful Family Resource Center. Nearby Reading Terminal Market makes a good lunch stop.