I still use size large sloomb organic bamboo fitteds on my 4 y/o at night. He's 43ish lbs, and is in a size small boys. So, they will definitely fit past the weight limit. He's skinny, but not incredibly so. They have an XL size that is 32 lb +, but he's still got room waist wise in the large size. They give the measurements for waist/leg that may help you decide better than weight if they (or something similar) might work.
http://sloomb.myshopify.com/pages/size-chart