In the help pages of DDG, they describe how to enforce safe searching:
Please add the URL rewrite to support this to the safe search Web Filter toggle.If you wanted to force safe search on (or off) you could use our URL parameters and automatically append &kp=1 (strict), &kp=-1 (moderate - don't show explicit images), or &kp=-2 (off) to all duckduckgo.com pages.
Thanks!