Like most people, I use Google as my primary search engine. It is arguably the best meta search engine available. The problem with Google is that it takes some experience to understand how to search effectively.
For students, this is difficult. Instead, try a search engine called Clusty. Clusty categorizes searches for you. This can help a student narrow results as well as show them how topics are organized. This is especially helpful when beginning research.
What other search engines do you like for kids?