If you have been there, you need to visit at least once. They have a rotating exhibit, so you may want to call first to see what the main exhibit is in the library. There are also other buildings to visit as well. Quaker Meeting house, blacksmith shop (sometimes there is a blacksmith working), birthplace cottage, gravesite, prairie land and schoolhouse. There is a statue of Isis on display outside. There are boardwalks around the buildings. There is no charge for school groups. You only pay entry fee to the library museum. It is free to walk around the park.
Be sure to visit the park service building connected to the post office. There is a ranger program for kids. There is a booklet for the kids to answer questions. When they are done they get a ranger badge and can stamp their booklets with a special stamp. A great way to get the kids involved in the tour.
Here is a great site for students to use when doing research: http://www.ecommcode.com/hoover/hooveronline/