My middle daughter, is SO rough on her pants - she is constantly wearing holes in the knees.
Rather than just toss them out, I have been saving them to turn into cut-offs for this summer - but I decided to 'dress them up' a bit.
One pair of pants...cut to the desired length
Some fabric - (This was 45" across, but for bigger kids... or bigger ruffles...you could use 60" fabric.)
I cut two 4" wide strips and folded them over into 2" strips. Then I sewed two rows of loose threads side by side and I pulled on thread to 'gather' the fabric into ruffles. (Use strong thread or it will break!)
Keep gathering until it is the same length as the opening of your pants leg.
Then sew right sides together. I used several rows of the zig-zag stitch (to prevent the fabric form unraveling.) It isn't pretty...but this part won't be seen anyways.
Then flip this part under and top-stitch near the seam for a nicer finish
Do the other leg and ....and...