Acquiring robust semantic representations of numbers is crucial for math achievement. However, the learning stage where magnitude becomes automatically elicited by number symbols (i.e., digits from 1 to 9) remains unknown due to the difficulty to measure automatic semantic processing. We used a frequency-tagging EEG paradigm targeting automatic magnitude processing in children (N = 33, 5-10-year-old, predominantly of White or Mixed ethnicity). A stream of digits (1-9) was presented at 10 Hz with small digits as standards, and large digits as deviants at 1.25 Hz (and the reverse). Frequency-tagged responses to the deviants show that magnitude is a salient semantic feature associated with digits in long-term memory. Automatic access to magnitude can be measured with EEG in children at the end of preschool.