The Bog Baby Book by Jeanne Willis and Gwen Millward

When two small sisters go fishing to the magic pond, they find something much better than a frog or a newt. They find a bog baby. Small and blue with wings like a dragon, the girls decide to make him their secret. I won't tell if you won't. But the bog baby is a wild thing, and when he becomes poorly, the girls decide they must tell their mum. And she tells them the greatest lesson, if you really love something, you have to let it go.