A book cover is supposed to be the most important thing of any book and while choosing any book cover designer, you must look at the number of books the person has designed. You must also look at various designs that the designer has produced. Once you are satisfied with the work of the designer, you must explain your idea about your book cover and in case you are not clear then ask for some help from the designer. You can explain to him/her the story theme and ask to make some samples. It may be a free sample or a paid sample depending upon the understanding of the twp of you.
Usually any designer will be willing to provide you with a sample at some cost, not the full cost for a design, but for some percentage of the full cost. You will be provided with a concept of the design and then you can share your ideas and reviews about the design and the right cover may be developed with some efforts. There are some very creative designers also who actually understand the theme so well that they can amaze you with the book cover design concept. If you get any such designer, consider yourself lucky.
Having the right book cover design for your book can prove extremely beneficial as everyone looks at the cover before making his/her mind to purchase it. A beautifully drawn cover which justifies the title of the book, can go a long way in selling your book. On the other hand, if a book cover is not consistent with the title of the book, and you feel that there is something lacking in the overall feel about the book cover, it can most certainly impact the sales of the book.
It is always advised to choose the right book cover designer who has the right kind of experience and many children book illustrators do this job very well as they know what is to be done with the text inn relation to the picture on the book cover. They have a good understanding of various colors as well and how to set a particular background color against the text. They have good know how in relation to designing good book covers.