It depends on the quality of the book, the child's interest and the ability of the child to respect the book. If you are asking for the sake of knowing what to charge for your book, I'll tell you an experience.
My partner and I were looking for a possible publisher or producer of our book and found a local one who does mostly nature-themed books. He wasn't interested in taking on our project, but he did answer a lot of questions for us.
One of our questions was about how much should we charge for our book. He told us he would put the retail price at about $9.95. (USD) We thought that might have been a little high, but he said that people who buy books for children-- especially grandparents-- were usually willing to pay a little more than they would for another kind of book.
Also, he said, that you can't really raise a price once it is set, but you can always cut it. Also a little higher price will give you some leeway to offer it to resellers at a wholesale price and still make a little profit.
He was right. And it was one of the most valuable bits of advice we got anywhere.