One knees of her pants had a 4+ inch slit, frayed, etc. In fact it was so frayed that I could not simply "join" the two sides together--when I had trimmed it there was quite a hole. The other knee was also extremely thin, so I figured it would go at any minute.
Actually the entire pair of pants she inherited from her brother is really quite worn out, but we all love the pants and she wanted them repaired. They probably won't work for her next year (except as the zip-off shorts).
On the back of each knee I used iron-on patches, then I applied the decorative stuff her mommie bought to the front side. It was mandatory to cover the ENTIRE slit on the one knee, and fortunately the patch had the square, plus quite a length of the little trees. It lent itself very nicely to the repair.
The worn thin knee
The knee with the big slit
(I should possibly not mention that my CRAZY daughter spent over $8 on the patch kit!)