While there are words like "grainlet" and "granule," these are more specific to small particles of something, rather than a smaller version of a grain of wheat, rice, etc.
If you're looking for a way to convey a small amount of grain, you could use:
* A little grain: This is straightforward and emphasizes the small amount.
* A bit of grain: This is similar to "a little grain."
* A sprinkle of grain: This implies a very small amount.
* A pinch of grain: This is more specific to very small quantities, like a pinch of salt.
Ultimately, the best way to convey the idea of a small amount of grain will depend on the context of your writing.