Boolify is a fantastic search engine designed for children to use. It helps them develop their information literacy skills by becoming familiar with Boolean searching.
It illustrates how each change they make changes their results. These results are filtered through Google's Safe Search STRICT technology but as they say "no filtering technology is 100% secure".
There are also some Word documents to download and use in the classroom which have some activities about evaluating web sites, basic Boolean searching and refining search results. Currently being developed are documents about Venn diagrams.