Objectionable content like offensive language, sexually explicit or violent material, gambling, or hate propaganda can be avoided by allowing children to only use kid-friendly searches. Results from searches are filtered by bots and some sites retain an editorial staff that reviews the content from each site.
Children's Search Engines:
Google SafeSearch for Kids
Online Safety for Kids:
FBI: A Parent's Guide to Internet Safety
National Crime Prevention Council
FEMA for Kids
FTC: Kidz Privacy