Formal:
* Ychydig bach: This is the most direct translation and is a good general option.
* Yn unig ychydig: This emphasizes the "only" aspect.
Informal:
* Dim ond ychydig: This is a more casual way to say it.
* Braidd bach: This is a colloquial expression meaning "just a little bit."
Example Sentences:
* Dwi eisiau ychydig bach o siwgr yn fy nhê: I want a little bit of sugar in my tea.
* Mae hi'n rhy boeth, rydw i eisiau yn unig ychydig o ddŵr: It's too hot, I just want a little bit of water.
* Dwi'n hoffi'r ffilm honno braidd bach: I like that film a little bit.
Remember that the best way to say "only a little bit" depends on the context of your conversation.