Learning to read in most alphabetic orthographies requires not only the acquisition of simple grapheme-phoneme correspondences (GPCs) but also the acquisition of context-sensitive GPCs, where surrounding letters change a grapheme's pronunciation. We aimed to explore the use and development of simple GPCs (e.g. a [right arrow] /ae/) and context-sensitive GPCs (e.g. [w]a [right arrow] /[open-mid back rounded vowel]/, as in "swan" or a[l][d] [right arrow] /o:/, as in "bald") in pseudoword reading. Across three experiments, English- and German-speaking children in grades 2-4 read aloud pseudowords, where vowel graphemes had different pronunciations according to different contexts (e.g. "hact", "wact", "hald"). First, we found that children use context-sensitive GPCs from grade 2 onwards, even when they are not explicitly taught. Second, we used a mathematical optimisation procedure to assess whether children's vowel responses can be described by assuming that they rely on a mix of simple and context-sensitive GPCs. While the approach works well for German adults (Schmalz et al. in "Journal of Cognitive Psychology," 26, 831-852, 2014), we found poor model fits for both German- and English-speaking children. Additional analyses using an entropy measure and data from a third experiment showed that children's pseudoword reading responses are variable and likely affected by random noise. We found a decrease in entropy across grade and reading ability across all conditions in both languages. This suggests that GPC knowledge becomes increasingly refined across grades 2-4.