Cover for a book written by a small child for his mother

I am working with my four-year-old son to write a book for his mother, about dragons. I would like a nicely designed cover. I really don't know how many hours it takes to create something like this.

Here are three reference examples of the style of books he is looking to mimic

http://3.bp.blogspot.com/-zLM9k1XJGDU/Utiw4kYwCqI/AAAAAAAARZE/tLgO9qD5Lvc/s1600/myfathersdragon.png

The title of his book is "The Dragons of White Perine". I would like the cover to feature two cartoon dragons who are visible dizzy and/or disoriented, in a forest. The dragons should be friendly looking. Beyond that, we are open to artistic license.

Can you potentially quote me for how long you think this would take, both in turnaround time and hours?

Thank you, you will be making my son and wife happy!