This is an easy one for me. I have a 3 and 5 year old, and we have computer time once a day. We usually go to the following sites:

http://kids.yahoo.com/games/games-index

http://www.fisher-price.com/fp.aspx?st=2601&e=gamesByAge&mcat=game_toddler&sit e=us

http://home.disney.go.com/games/index

http://www.thomasandfriends.com/usa/play_online_games_with_thomas_and_friends.htm

http://vtechkids.com/

http://www2.leapfrog.com/do/environment?environment=leapster&ageGroupKey=presc hool

All of these are GREAT sites. It's just a matter of what your kids are into! Have fun!!