You may want to grab a pair if Asics GEL kneepads, they are thin, light, stretchy and squishy. They are intended for wrestling (freestyle/folkstyle, not professional). Many of my teammates and I wore them in High School. If they can handle the beating of seasons worth of take downs and sprawls, they can handle a little suwari waza. They also should give you no problem under your gi as they are basically neoprene and a gel pad. Additionally, they are cheap - in the USD $12 - $15 dollar range.
If you get them, do your uke a favor and wash them occasionally, Those little puppies can get to reeeeeeekin sumptin' fierce : )