While much is documented and theorized about how children acquire the meaning of small numbers expressed in simplex numerals (number words that cannot be decomposed further), relatively little is known about how children come to understand complex numerals (that are composed of simplex numerals). We aimed to investigate this developmental phase with a hypothesis that children learn that complex numerals are syntactic compositions of simplex numerals, prior to understanding the meaning of the complex numerals. In a novel task, children compared the numerical values of two large numbers that differed only in one syntactic position (e.g., four hundred twenty chairs vs. six hundred twenty chairs ). Together in two studies, with Korean- and English-speaking children, we demonstrate that children with limited count list nevertheless are able to compare large numbers and that this ability correlates with their counting fluency. At the same time, children find it more difficult to compare two complex-numeral phrases (as the example above) than two simplex-numeral phrases with the same number of words (e.g., four little yellow chairs vs. six little yellow chairs ), and we demonstrate additional evidence that syntactic complexity influence comparison performance, suggesting a cognitive cost to parsing the syntax of numerals. The findings contribute to the counting literature by contextualizing children's understanding of the count sequence in the linguistic theory of numerals. We propose that children, as they learn to count, develop an implicit understanding of numerical syntax which may be one of the first steps to acquiring generative number concepts.