Mr. Chris happened to find a cool blog with a really cool tip, reinforce those knees BEFORE they get trashed. http://thefrugalgirls.com/2010/01/how-to-prevent-knee-holes-in-jeans-and-pants.html
I took this one step further because there was no way i was going to spend almost $2 per pair of jeans when i can get a 17 x 36 piece of thermabond for $2.45/yd. (that is nearly 30 4x6 patches!) I got the heavy duty no-sew stuff for applique work. I bonded it with my iron to scraps from a pair of my jeans that were just plain worn out...almost more hole than jeans! and took care of knees on 8 pairs of kid jeans with one leg, and still have plenty of material and thermabond left over.
only tip with that method: make sure you round the corners on your patches so they don't peel up! and try not to dry jeans on hi heat, always safer to go with low.