My wife and I have three kids - the eldest are twin girls, four years old. And they've decided to take it upon themselves to begin writing books. One of them just finished her book today. It's four regular sheets of copier paper front and back, with art on the top third and writing on the rest. We had to help her spell a number of the words, but the story itself was completely hers, every word of it from her brain. She took the completed pages, borrowed a roll of tape, and taped the pages together. Then she taped two pieces of black construction paper together as a cover, adding cover art on both front and back covers, and taped the pages into the cover.
The result is actually a surprisingly good binding! =)
Her story is a tale about a girl, a unicorn, and a butterfly on a short adventure together. It's a short story. Her handwriting is pretty typical for a four year old, and she's just begun grasping the concept of what a period is. But it's her story - her book - and it's now sitting on the kids' bookshelf with their other books. She's intensely proud of her effort, and so am I.