Your app looks great, but your QA may need to be stepped up a notch.

Overall, although your app seems great (purchased it recently), there is at least one serious and extremely obvious bug. The spelling quiz feature relies on a voice synthesizer. The British accent (haven't tested the others) version fails totally on the first word (easiest) spelling list, and the short words produced are unintelligible. It would seemingly be easy to have tested this, and I'm surprised this wasn't picked up.

I'm not picking on you, and I'm glad I purchased your app for my child. However, this bug made me wary of buying your other app.