(You may want to start here if you know your focused topic.)
Tekmom--This contains multiple search engines. Try several of them before you are finished with your search. Using different search engines means different results.
Kids Search Tools (a meta-search engine)
Kids' Click--indexed by librarians
Awesome Library--reviewed resources
Pandia Search Engines, Directories, and Collections page (This is not a search itself, but it links to many different ones.)