I had a bit of an issue with the age categorisation for children, not having children or knowing any children i just had to go from my own ideas and from various articals
http://writeforkids.org/2014/02/understanding-childrens-book-genres/
http://www.scholastic.com/parents/resources/article/parent-child/about-our-age-groups
I think the consensus is for younger children the books are bright with high contrast and possibly tactile. the pictures seem to get more complicated and somewhat more realistic for older readers.
I think there also some cross over for example where spot i would consider showing to a baby as well as a pre school child.
I dont think that the age ranges are as appropriate just more of a guide line children learn at different rates and have different tastes, they are after all mini people. with the likes of some books harry potter or the dark materials series they cross over into adult genre as well. for example with the harry potter series there was an adult cover and a children's set of covers (I bought the children's because they where much more appealing to me!) when is started brain storming i initially thought of an elephant and festival and then researched Ganesh and the festival where elephants parade the street.
i would be tempted to repaint it digitally as it didnt come out as well as i though with watercolours, gauche and ink. however i do like the overall design and the colours they are bright but not massively in your face bright so i hoped they would appeal to a wide range.