Our daughter is no stranger to slates. She is pretty much the primary user of our iPad as she finds it more useful than I do.
So tonight she saw my Kindle Fire on the couch. She grabbed it, said where is the button. She quickly found the power button and within 10 second was playing Angry Birds.
However, she has spent the last minute looking for the volume button. And she can't figure out how to exit the game.
My observation is that the iPad is a lot more intuitive for a 5 year old.