There are several kids educational sites out there for free.
One of them is Starfall. This is geared towards students in preschool through early elementary and has a variety of activities for them to do online without even having to sign up for an account. Starfall began with a focus on reading, but has expanded and now has math activities. It teach them shapes, colors, calendar concepts and much more. This is a site that my cohort of literacy coaches reviewed a couple years ago and gave high marks for its cost and content. I've recommended it to friends with young kids. Even now, my niece just saw me write this and said that she uses this site in school. She wants to go on it when I'm done working....so, it's kid recommended and approved too!
A second one is Fun Brain. This site has games for students in grades K-8. Again, students do not have to register to play. They simply identify their gender and grade level. The site is designed to be highly engaging, using games to teach math and graphic novels to engage them in reading. The book, Diary of a Wimpy Kid, by Jeff Kinney is posted on it. The Lost Side of Suburbia, a slightly scary graphic novel by Kory Merritt, is also on there. There are also a variety of madlibs. This site has done its research and uses activities that are highly engaging to encourage students to play and learn at the same time. My nephew in upper elementary gives it two thumbs up!
Finally, there is PBS. This site has a wide range of educational games for a variety of subjects and topics. The site is very comprehensive and easy to navigate. The range of activities is expanding all the time. And again, all the kids I know like this site, using it at school and home.