Children's early math abilities have been claimed to depend differentially on various cognitive competencies (e.g., core number skills, working memory, and general math and reading abilities). Clarifying the relative importance of these different cognitive markers in predicting key early math skills would provide a conceptual framework for understanding development of math competence. To this end, we assessed 267 six-year-olds' core number (dot enumeration, number comparison) abilities, visuospatial working memory, and standardized math and reading test performance, as well as their number fact retrieval speed and ability to transcode number strings (e.g., "4,073") at 6, 7, and 8 years. To minimize interpretive difficulties associated with previous analyses, we used a random intercept cross-lagged panel model to characterize the reciprocal relationship between number transcoding ability and number fact speed over time. We tested the hypotheses that general math scores would predict individual differences in number fact speed, while general reading scores would predict individual differences in number transcoding accuracy. Findings showed that both standardized tests and core number measures differentially predict math skills in the early school years. Dot enumeration and general math ability predicted individual differences in number fact speed, while general reading and general math predicted individual differences the ability to transcode number strings over time. We suggest combining standardized assessments of math and reading, together with measures of core numerical ability, at school entry offers important information about the factors associated with key early math skills.