I read this blog this morning ... http://www.guardian.co.uk/lifeandstyle/the-womens-blog-with-jane-martinson/2011/aug/17/diet-book-for-girls
... and my answer to the writer's question is No. It's not a good idea.
Disclaimer: I have not read the book. However, in reading the book's description (posted here: http://bit.ly/rfZd8u) there's a premise I have to disagree with: You do not have to be "a normal sized" person to be confident or have a good self image.
The book cover is disturbing ... as the girl does not see herself as she is in the mirror, but a skinnier version of herself. Honestly, that is way to much pressure for a child. And kids have way to may pressures put on them these days.
Parents, caretakers .... there's some stuff you can do NOW to address a child's weight issue ... if the child has a weight issue (remember not everyone is meant to be a size 0):
Make sure your child gets plenty of exercise. Make exercise a part of life -- for the long term.
If you do not want your child to have a weight-related health issue, feed him/her the right stuff. Too many of us are quick to feed kids happy burgers and fries, too many pizzas. And, we happily do this everyday, at the end, to our own chagrin. Make this a family affair. YOU need to set the tone. But, most of all, do not make your child (and do not let anyone) feel that they are less because of their size.