Recycling 101; take your pants with the worn out knees and make them into shorts. This practice of re-using pants to make shorts is especially nice for kid’s cloths due to the tendency for children to prematurely wear holes in the knees.
Cut at the desired length (add extra if you want to leave a cuff).
Sew a hem, add a cuff or just leave it raw so it will fray.
These shorts were made from a pair of pants that my daughter loved. They had developed holes in the knees so I cut them into shorts and rolled up a cuff and hemmed the new edge. In addition, I cut out the flower detail that was at the bottom of the pants and re-sewed it on the shorts. It looks like I bought them this way.