The Aquarium in Camden is really nice as is the River Sharks Stadium when they play. Young kids don't really know the difference between them and the Phillies, and they just like being out in the ballpark where they can cheer and eat junk food.
Good Call on the River Sharks! I had forgotten about that...we went to a game last year and had a blast. a fraction of the cost of going to see the Phillies, and VERY family friendly (great, close-up seats, they let the kids run the bases before the game, carousel in the stadium, and often offer really good deals on family packages online...we probably spent less than $50 total for 4 of us including tickets, parking, lots of food/snacks in the park, and carousel rides. The game was great/exciting too! Highly recommend!
Independence Mall where they can see the actual birthplace of the United States, the Liberty Bell, and if your kids are mature and adventurous enough there is Chinatown. which has excellent restaurants.
10-11-12 would probably be a good age. There are several good restaurants, and about an equal number of Asian grocery stores worthy of browsing if just to see the different kinds of food that you will never see in a Shop Rite or Walmart. Also a few other Asian businesses like travel agents, beauty salons catering to Asian tsastes. It's fairly small compared to NY, about 4 blocks by four blocks. One of the streets has a big carved arch over the street known as the Friendship Gate. It's a visually stimulating and aroma filled place, educational at the least. My wife is Asian, so we always stop in there when in Philly, for the food, especially groceries that we can take home. Asian candies are quite different than the typical American stuff if your kids are adventurous enough to try flavors like mango, and papaya.