Research has demonstrated that providing labels helps children notice key features of examples. Much less is known about how different labels impact children's ability to make inferences about the structure underlying mathematical notation. We tested the impact of labeling decimals such as 0.34 using formal place-value labels ("3 tenths and 4 hundredths") compared to informal labels ("point three four") or no labels on children's problem-solving performance. Third- and fourth-graders (N = 104) learned to label decimals while playing a magnitude comparison game and placing decimals on a number line. Formal labels facilitated performance on problems that required understanding the role of zero. Further, formal labels led to lower performance on problems where a whole-number bias led to a correct answer, suggesting that formal labels may have reduced a whole-number bias. Overall, formal labels helped highlight the place-value structure of decimals, indicating that labels can help children notice mathematical structure.