Litnerd - The Pixar for education, reading program for K-5 schools
CurrentWare BrowseControl - BrowseControl web filtering software blocks websites based on URLs and categories. Policies can be customized for each user, department, or OU. Includes other features such as an app blocker, port filter, and download/upload blocker.
Pili Pop - Great language learning method for kids
Qustodio - Free parental control software. Monitor and track your child's online activity. Block dangerous sites and protect kids from online bullying.
Wikids - An encyclopedia that talks to your kids
NxFilter - Best performance. You don't want to slow down your network by installing a network security software. NxFilter is a light-weight DNS filter. It doesn't cause any latency issue in your network.