It is the Birthplace of our Nation. There are hotdog and pretzel vendors on just about every corner. There are lots of free and low cost fun things to do. South Philadelphia is the only place to get a REAL Philadelphia Cheesesteak . The Liberty Bell and Betsy Ross's House are here.
Public transportation makes it easy to to get around in the city and out to the suburbs.
It is a great walking city. Philadelphia has the oldest Zoo in the country. A special Guests and Kids Tour on Tuesday lets you attend your meetings and programs while your family heads to Great Adventure and Safari park for a day of fun and memories. The AALL locals recommend a few of their family's favorite places. The Family Friendly Philadelphia On-line Guide provides a great list of things to do with the kids.