See, everything is a toy to a six year old. Everything they see, hear or handle shapes their development in some way. Devices like smartphones and tablets have great unique uses, like people in other comments are saying, so I'm not suggesting you ban kids from using them or anything.
But I maintain books have far more physical qualities than any tablet or e-reader. Variety, for starters. Tabs and e-readers all come in some shade of black or gray, they all present a very similar glass texture to the touch, and they only have that new gadget smell for two days. On the other hand, very few books are the same. Moving from a magazine to a hardcover to a paperback may be all the same for you and me, but for a 6 year old is an extremely important feast of sensations that informs her developing mind about the myriad materials and textures and smells the world is made of.
Why rob a child of that variety?