Here is a list of some of the more popular filtering programs or services to prevent minors from viewing adult content. I do not endorse or otherwise recommend any particular one:
SurfWatch
Net Nanny
CyberPatrol
CyberSentinel
Cybersitter
Internet Filter Suite
Kiddonet
Mama Bear
Safe Surf
18 U.S.C. 2257 Record-Keeping Requirements Compliance Statement