I would second what was stated above and suggest the National Constitution Center. I've taken school kids there before and they really enjoy it.

If you go to the Betsy Ross house, Elfreth's Alley is right around the corner, plus that section of the city (Old City) has a lot of interesting, unique shopping; some of the best are located on 3rd street between Market and Race and also on 2nd street.

One other strong possibility for that age group would be to take one of the Duck Tours. Lots of zany fun and informative.