Knees seem to be one of the first things to wear through on a boy's garment. To add durability as well as rugged styling I like to add a reinforced knee to boys pants.
1)Cut a piece of your fashion fabric so that the length equals the width of the pant leg. I've found that 6 inches works well for the approximate width of this piece, but you'll need to experiment with wider or narrower to match the proportion to your garment size.
2)Lay this piece of fabric so that it is centered where the knee will fall in the pant leg. Trim the edges to match the pants side seam and inseam.
3) Press under the raw edges and then pin back into place.
4)Topstitch the piece in place.
Optional: You could place a piece of batting under the knee band if you wish to make a padded knee. Great for little ones scooting their trucks around on the floor. You could also quilt in various design lines onto the band if desired.