How about a Very Baby Simply Nights? bamboo is available atm, or hemp works well too. I know it's a night nappy, but it would probably be reasonably trim on a 2 yo. My nearly 2 yo wears them at night and I don't find them bulky at all anymore. You can get side snapping too, which means you can adjust the legs and waist seperately to accommodate chunky thighs
I think they are about 14 layers of whatever fabric you get, so fairly absorbent, just more surface area than the nappies you've already tried. might work.