I love elbow pads & knee pads on pretty much anything for kids, there's a geek quality!
I got a pair of colorful tights at, you guessed it, Target! I was worried that since they weren't made of cotton that the paint wouldn't stretch well it ended up being perfect!
use a can of some sort to stretch the leg out and position your oval at the knee.
use a marker, draw an oval and repeat to the other leg, make sure that they are the same size
Next paint your oval, I used two tones, one was a brown & one was a gold. it gave it kind of a leather looking effect. Let the paint dry while the can is still inside and voila!