The first thing that wears out is always the knees!
I don't like the look of the iron on patches that go on the outside of the pants.....so, this is my solution.
Grab some scraps of fabric, fray block, and fabric glue.
Start by putting some Fray Block around the perimeter of the hole. This will stop the hole from fraying any bigger. Use a small amount and be careful because it is runny stuff.
Let the Fray Block dry.
Next, put some plastic up the pant leg to protect the leg from gluing to itself.
Cut a piece of fabric larger than the hole. Use pinking shears if you have them because this will help prevent fraying.
Insert the fabric into the pant leg (on top of the plastic) and position it under the hole.
Working from the outside, lift the edge of the hole and put a thin line of fabric glue around the perimeter of the hole. Let Dry. That's it!
Sometimes I take it one step further and cut the pocket out leaving a good 5/8" around the perimeter so you will have something to glue your fabric to, and for that "frayed" look.
Wash them, and your done. It is like a new pair of jeans. My daughter gets tons of compliments!