Here are a few options you could use, depending on the context:
* Pōhaku liʻiliʻi: This literally translates to "small stone," but it's a common way to refer to a little rock.
* Pōhaku ʻuʻuku: This translates to "small rock."
* Pōhaku liʻiliʻi ma kahakai: This would translate to "small rock on the beach."
Ultimately, the best way to say "little rock" in Hawaiian would depend on the specific context of your conversation and what you want to emphasize.