Check this link. It is for toddler shorts, but you can add the length needed. There are others available, but this is the quickest I could find for you. I hope this helps. Good luck! http://creativezaida.freeservers.com/shorts.html
Make one using your toddler. Take some newspaper, taped together so it is bigger than your toddler. Try to get your little one to lay down on it. Trace around your little one. Give an extra 2 inches on the bottom to hem the legs. Given an extra 3 inches to put in an elastic or draw string waiste band.