This is a great question. Let me make a few points that might help you to see the nuance in view.
First, before we talk about the difference, it is important to mention that there is a lot of overlap between the words, "only a little" and "a little." So, in most cases, they will be interchangeable. This should not surprise you, because from a linguistic point of view, language overlaps.
Second, there is nuance. The word, "only" makes the emphasis on "a little" stronger. In other words, the adverbial force intensifies the words, "a little." Perhaps if we changed the word "only" with another adverb, we can appreciate the difference. We can use the word, "exclusively," or "solely." As you can see there is a sense of definiteness.
Third, without the word, "only" the emphasis is diminished a bit. However, it is important to keep in mind that they are interchangeable for the most part.
Only a little & a little are interchangeable. "Only" gives emphasis/strength to the meaning