Here are links to some sites that are designed just for kids!
All Subject Sites
Sheppard's Software http://www.sheppardsoftware.com/
Moby Max http://www.mobymax.com/signin
ABCya http://www.abcya.com/
Scholastic Interactive http://teacher.scholastic.com/activities/clf/tguidesitemap.htm
Search Engines
safesearchkid.com (filtered search)
mel.org (Michigan Library)
Merriam-Webster (visual dictionary)
Search Engine (Top 20 search engines for kids)
Word Central (Children's dictionary, thesaurus, and rhyming)
NWEA
MAP Math Activities http://www.sowashco.k12.mn.us/ro/pages/studentlinks/map/default.htm
MAP Reading Activities http://www.sowashco.k12.mn.us/ro/pages/studentlinks/map/reading.htm
MAP Reading and Math http://www.belleplaine.k12.mn.us/page/3028
Science
ScienceZone (interactive) http://resources.woodlands-junior.kent.sch.uk/revision/science/
Why Files (interactive) http://whyfiles.org/category/interactives/
Weather (interactive) http://www.edheads.org/activities/weather/
Sun, Moon, Earth (interactive) http://www.earthsunmoon.co.uk/
Food Chains (interactive) http://www.sheppardsoftware.com/content/animals/kidscorner/games/foodchaingame.htm
ScienceNetLinks (interactive tools, click grade and click search) http://sciencenetlinks.com/tools/
Social Studies
Scholastic News Fourth Grade http://sni.scholastic.com/sn4
Scholastic News Third Grade http://sni.scholastic.com/SN3
Underground Railroad Virtual Tour http://education.nationalgeographic.com/education/media/underground-railroad-journey-freedom/?ar_a=1
Ellis Island Virtual Tour http://aam.govst.edu/projects/rharrison/student_page2htm
First Thanksgiving Virtual Tour http://www.scholastic.com/scholastic_thanksgiving/daily_life/
Ben's Guide to Government http://bensguide.gpo.gov/3-5/
National Geographic Kids http://kids.nationalgeographic.com/
Westward Expansion http://globalgamenetwork.com/westward_trail.html
Sims Sweatshop http://www.seimsweatshop.com/game
Math
Illuminations (common core math games) http://illuminations.nctm.org/
Cool Math http://www.coolmath.com/
AAAMath http://www.aaamath.com/
Think Central http://www-k6.thinkcentral.com/ePC/start.do;jsessionid=E7E32FA388D5AE94966F0D97BF31C728
Language Arts
English Language Learners (individual and classroom games) http://www.eslgamesplus.com/
Webtales (like Madlibs) http://www.eduplace.com/tales/
Word Hippo http://www.wordhippo.com/
NG Connect (Reach for Reading) www.myngconnect.com