I love Robin McKinley--I think she is one of my favorite authors. Typically she writes fantasies that combine an interesting setting with a good plot and stirring characters. Normally, she doesn't write high fantasy. Sure, she jumps around a lot in a narrative and sometimes her pacing is downright weird, but usually there is a huge payoff so it's all worth it. In the book Pegasus... not so much.
Pegasus takes place in a world where humans live side by side with huge winged sentient horses, although most humans aren't able to communicate with the pegasi without the use of an interpreter. Sylvii, a princess, is "bonded" to her Excellent Friend Ebon on her 12th birthday and discovers they can communicate telepathically, which causes quite a stir in the kingdoms.
Pegasus is a very serious book. I don't mean that it deals with adult issues or anything like that, but it is not really written from the perspective of a 12/16 year-old. It reads more like an adult novel about a child. The narrative jumps around constantly so it's a very demanding read and the pacing is very, very slow. Normally I can handle this but the payoff just wasn't there for me. The ending is very abrupt and apparently this is going to be part of a series because the book totally breaks off without warning. I didn't find any of the characters particularly compelling--they lacked depth.
I think this would be a difficult book for children to get through, however, if they really loved horses it might be worth it because a lot of the book is about how great the pegasi are and how cool it is when they fly, etc...
This book reminded me of The Two Princesses of Bamarre by Gail Carson Levine, a book I would highly recommend for children and adults both. Both books are about princesses that have adventures but Two Princesses has better characters and plot.