Advice on good educational apps

Long story short, I bought a tablet to provide my kids with some good educational apps.

I've installed Duolingo, Khan Academy, Google Earth and one or two more.

But I'm appalled at how hard it is to find interesting apps with quality content. I don't mind paying if it's worth it. The "education" section of the store is basically a scam. Everything that is kids oriented, even if its just some junk cartoon-y app, gets classified as "educational".

So, resorting to real people for advice: can you give me some pointers to some good stuff? :)