My eldest boy has a thing with wearing the knees out of his jeans. Maybe it’s because I buy used clothing. At any rate, this is how I take care of the problem.
1. Start by ripping out the inner leg seam. For a small pair of pants, you practically have to rip from the crotch down to the hem, but not quite. I ripped from pin to pin on this pair of jeans.
2. It will look like this when turned right side out.
3. Cut a large patch from scrap denim (I use old jeans). You want it to be generously sized so it reaches to good, unripped areas of fabric around the hole. Pin it on to the exterior, making sure the side towards the inner leg seam will get caught in the seam when you see it back shut.
4. Use a close zigzag stitch to sew around the three edges of the patch that will not get caught in the inner leg seam.
5. Next I like to use a straight stitch to sew random straight lines all across the patch. This reinforces the patch and keeps the hole from ripping further.
6. Turn the pants inside out and pin the inner leg seam shut.
7. Sew down the original stitch line, making sure to overlap the previous stitching that you didn’t rip out.
8. Serge or zigzag stitch the raw edge of the seam so it doesn’t fray.
9. Finished seam:
10. Finished patch:
11. Happy boy!
12. Alternatively, place the patch on the interior of the leg before sewing as shown. It looks decorative and trendy!
ZoeDawn on Etsy 
My sister used to reinforce the knees of new jeans from the inside. Use either iron-on patches, or steam-bond pieces of old jeans inside the legs at knee-level. The jean knees will wear much longer.
What a good idea! Thanks!