These pants have been waiting in the mend pile for over two years.
I sewed some fleece fabric to the inside of the knees first and zig-zagged all around the holes so it wouldn't keep fraying with every wash & dry. Then I cut out a couple of 5-inch squares from some grey corduroy. The corduroy is fairly thin, so that's why I used fleece on the inside; to give it more stability and also add a little bit of cushion for this little boy's knees.
Eli's wear-and-tear eventually ripped out the original knees.
Now Reid's wear-and-tear can eventually rip out the patches.