Way More Than Math
Yes, Khan Academy is great for learning math; from Early Math (about K-2) to college level. You can take an organized approach, or just keep doing mastery and practice tasks and get hints or watch a video on the topic when you get things wrong. But there is much more than math! One of the biggest parts of Khan Academy is coding, especially using ProcsessingJS. Now, in addition to JS they have HTML and SQL , and courses on how to combine JS and HTML to make interactive websites. Mind you, this can be some rather complex math (geometry, fractions, and the like) so your Kindergartener would likely be unable to do more advanced coding but it's great for older kids in our increasingly-digitalized world. Not to mention you can browse other people's programs and vote them up, leave questions about how they work, or leave tips or congratulations. (Be warned though, some people will make replications of popular games such as Five Nights at Freddy's). There are also lessons on various fields of science, music, U.S., World, and Art History, grammar, economics, and there's a test prep section as well. Now, in science things like the theory of evolution and global warming can be controversial, not to mention reproduction and various other systems are explored which may gross you out and the fact of the matter is, history lessons will cover war and conflict. The complexity of some subjects make most of the site better for older kids, but young children can still learn about math and grammar. Oh, and best of all, the site is free and ad-free as it runs on donations.