I read it to my 4-year-old and he wanted me to read it again and again, which I'm sure is great news since he's the target audience. :-) The pictures made him laugh--especially the hippo trying to peck-peck-peck the watermelon. The grammar and spelling are clean. I was able to read the book aloud smoothly, so the words fall together perfectly. My son definitely had a basic understanding of what the book was about and once we finished we had a discussion on how important it is to do what he likes and not what other people like. He did understand that the hippo was trying to make others happy which did not make the hippo happy, so he did understand the message. He only wants books he really really likes repeated. Usually, if we read three or four books for the day, he wants his favorite repeated. There are plenty of books he has no interest finishing from start to finish, so he knows what he likes. :-) His bookshelf has about 50 books and there are three books that he wants read over-and-over, so hopefully that gives you an idea of how much he liked your book.
I read the book five times (four times with Ayden) just trying to find more to critique, but the book was so good that it ended up just being fun to read with my son.