Post to the Facebook group, Westminster Theological Seminary Student Community. This FB group, closed but to current students and their spouses, prospective students and their spouses, and current employees and their spouses, is a great way to move items within the seminary community.
Consider donating books to the Christian Literature Crusade. The organization's international headquarters, and drop-off point for Bible and Bible reference books, is quite nearby in Fort Washington. They provide an address for shipping other books. See here for more info and Fort Washington drop-off address.
Consider donating books to the Theological Book Network. See here for more info, including direction on how to donate.
For baby and toddler items, please consider donating to Amnion Pregnancy Center. Such items will be put to good use by those who need them for their little ones.
New Life Thrift is a good option for other items. They give Westminster students and employees 10% off purchases! Just show your library card with your picture at check-out.
Please don't leave free goods anywhere on campus. The seminary does not have resources to be a receptacle for such items, and so the items are invariably trashed. Follow the steps above to put them to good use!