Here are some options depending on the context:
For small bites:
* Kain ng konti (literally "eat a little")
* Kagatin ng dahan-dahan (literally "bite slowly")
* Tikman (to taste, which can also imply a small bite)
* Subuan (to try a small amount, often of food)
For a gentle or hesitant bite:
* Mangagat ng dahan-dahan (literally "bite gently")
* Sipsipin (to suck, which can be used for nibbling on something soft)
For animals:
* Kinagat ng bahagya (literally "bitten slightly")
Ultimately, the best translation would depend on the specific situation and the type of nibbling you're trying to describe.