After allowing my grandchildren half an hour's free reign on the iPad (mostly simultaneous access), the weren't fingerprints on the screen, just a film of grease .
The favourite apps?
Space Invaders, it looks and sounds just like the original on the Apple II from the late 70s, as my children played it.
Google Maps, they were both enthralled by seeing where their house is, our house, great grandparents on the other side of the continent and the other side of the world. It was surprising how much they seemed to comprehend or perhaps just accept. Lastly,a big winner was satellite views of various airports with aeroplanes to be seen.