The biggest one early on was parents saying the free tier messaging was confusing. The
landing page said "2 lessons per day," but once they logged in, the dashboard said "8 Correct Answers Per Day." Two different metrics for the same thing. Parents didn't know what they were actually getting. I fixed that by making the messaging consistent everywhere,
landing page, dashboard, subscription panel, the lot. Lesson learned, if a parent has to think about what your pricing means, you've already lost them.
The other common one is parents wanting a proper mobile app rather than using the website on their phone. The site works fine on mobile; I made sure of that from day one, but parents expect to see it in the App Store. That's on the roadmap. For now, I just make sure the mobile web experience is solid and tell people to add it to their home screen.