The iPad doesn’t have a calculator app by default, which has caused a huge number of developers to try and fill that void. The best one? Calcbot.
Calcbot comes with a well designed interface and a few neat features like swiping down to reveal previous calculations, a live view of everything you type and sound effects resembling an old typewriter. You can also swipe from the right to access all the scientific functions for more complex calculations.
It has everything you’re likely to need in a calculator app, whilst still making the app suitable for all ages.