Earth, Sun, Moon

 http://www.kidsastronomy.com/dictionary.htm

 http://www.sciencekids.co.nz/sciencefacts/space/definitions.html

http://www.kidsastronomy.com/

http://www.esa.int/esaKIDSen/

http://kids.discovery.com/tell-me/space

http://www.planetsforkids.org/

http://www.seymoursimon.com/index.php/science_dictionary/

For pictures you can use:
 http://www.nasa.gov/multimedia/imagegallery/index.html#.Utge07yRN_I