I'm just wondering if any legumes are sweet. Preferably, I'd like a sweet bean, but whatever works.
Peas are very sweet, especially when you eat them young. I don't think any others reach them, but I don't think I've tried every legume in the world, so it's possible.
If the natural sweetness of legumes isn't sufficient for you, they take very well to added sugar.
adzuki beans are quite sweet and used to make various desserts in Asia.
I will note, however, that typically some form of sugar is also added in whatever dish is being prepared with them.