I remember Kids Discover from their print days- it was a well written (and ad free) periodical I purchased for my children. Now they have web resources. While you can purchase a subscription for total access, I’ve been looking at the free content- and it seems to match the same quality as the old magazine. Once you sign up for an account, you will also get weekly emails of free material for that week. It is a great resource for both science and social studies.