Young children have an overall preference for child-directed speech (CDS) over adult-directed speech (ADS), and its structural features are thought to facilitate language learning. Many studies have supported these findings, but less is known about processing of CDS at short, sub-second timescales. How do the moment-to-moment dynamics of CDS influence young children's attention and learning? In Study 1, we used hierarchical clustering to characterize patterns of pitch variability in a natural CDS corpus, which uncovered four main word-level contour shapes: 'fall', 'rise', 'hill', and 'valley'. In Study 2, we adapted a measure from adult attention research--pupil size synchrony--to quantify real-time attention to speech across participants, and found that toddlers showed higher synchrony to the dynamics of CDS than to ADS. Importantly, there were consistent differences in toddlers' attention when listening to the four word-level contour types. In Study 3, we found that pupil size synchrony during exposure to novel words predicted toddlers' learning at test. This suggests that the dynamics of pitch in CDS not only shape toddlers' attention but guide their learning of new words. By revealing a physiological response to the real-time dynamics of CDS, this investigation yields a new sub-second framework for understanding young children's engagement with one of the most important signals in their environment.