I would say
Rutgers Zimmerli Art Museum would be the best, fun, education and creative, all Piscataway schools 7 or 8 year old girls visited it and love it.
You can take NJ Transit from Midtown Penn Station to get off at New Brunswick Train Station, and just one block away is the museum.
Of course, you may also take the free campus shuttle bus to tour the best university in New Jersey that has Nobel Economics Prize winner,
Milton Friedman, in 1976, whose theory is the solution of current economic situation.
Across the museum is the Economics Department at New Jersey Hall (to be renamed Milton Friedman Hall, as I have proposed to President McCormick) where you can tell the great story of this Nobel Prize winner to your 7 year old girl guest. She shall learn a lot and inspired by all she saw and hear. Who knows, maybe some day she will come to Rutgers University study, and may be winning another Nobel Economics Prize some day.
You may take her over
Rutgers Campus Bus to Douglas College of woman that has glory history of American women fighting for the rights to vote. Just walk around one of the most beautiful college campus and music library and checking out student centers, you both shall learn a lot.
For a 7 year old girl, a train trip from NYC to NJ to famous university to top 3 university art museum in the US, and woman college, ...etc, will become one of the most inspired trip for her.