The little achievements and interactive bits were very effective at keeping him engaged. I'm not sure how much he's retaining, and for the multiple choice questions he seemed to just choose things randomly until he could proceed forward, but he definitely picked up at least a handful of random factoids that he'd tell us about later.
I couldn't have been happier about his interest in that instead of Youtube Kids videos. So much content for kids is just dumb stuff that is focused on keeping kids' eyeballs but not on anything else. The educational apps seem to be mostly focused on random simple math problems or spelling. Science-focused content for young kids has been practically non-existent, so we tried Tappity because it was pretty much the only option. We were very pleasantly surprised.
Thanks for making engaging content that I don't feel bad about exposing my kids to. Please make more content.