ClassontheWeb is a good beginning for school children. It offers excellent way of teaching the subject of secondary class students. The subjects are as per the school sylables. The explanation is done using simple graphics and the children can study with interest and involvement.
My son logs on to this site to study for his IX std subjects. He enjoys it thoroughly. It is also possible to have online chat with an expert teacher.
There are also online tests and children find this challenging as it does not come under the scrutiny of their other friends.
I encourage every parent to register their children to this site and experience learning online.