Comfyshoes-But I think one of the top educational websites for kids is Quia and Book Adventure. Quia offers a game show formats so that your child can learn various concepts.
For example one of their formats involves the I Want to be a Millionaire format in which the questions get progressively harder and are worth more money.
I sometimes use this with grammar concepts so that my kids have more practice. Book Adventure is another good educational website for kids that I use that offers quizzes on various books.
I try to find Accelerated Reader books at school that also coincide with this website so that they can get practice taking quizzes so that they can check their reading comprehension.
The site even offers a suggested list of titles depending on the grade level you choose. You can even select the list by genre. This is really helpful when my kids are reading chapter books that are especially long because they have multiple versions of all of their quizzes.
This site is a Sylvan site, but it is a completely free site that is one of the best on the internet.