My 7 and 9 year old daughters LOVE the series of "More..." apps. For the iPad, they have Cupcakes XL, which is how we first discovered theses apps. They received iTunes gift cards for their recent birthdays, so we've since downloaded More Pizza, More Salad, More Toast (kind of odd from a grown-up's perspective - but they like that one too), and they just released a More Sundaes. Not necessarily the most educational set of apps, but they love them.
My daughters also like the variety of art/coloring apps.
Hangman is another one that my daughters like. One thing they'd love is the ability to plug in their own spelling words into one of these hangman apps. Maybe there is one out there already, but we haven't found it yet.