In line with recent developments in both language acquisition and text comprehension studies, it is argued that more reliable and valid lexical richness measures can be obtained by including frequency class information. To that end, texts written by 452 elementary school children (L1/L2) in grades 3–6 were investigated. In order to find out whether a frequency class based lexical measure is more valid than type/token based measures, the central question to be answered was whether with increasing vocabulary skills from grades 3 to 6 (measured by standardized vocabulary tests), the number of low frequency words in children’s writings increased, and whether L2-children with lower vocabulary skills used relatively more high frequency words than their L1-peers. The results show a gradually growing number of low frequency words: children in grade 3 use more words belonging to the 1,000–5,000 word frequency range; in grades 4/5 more from the 5,000–12,500 range; and in grade 6 more from the 12,500-plus range. L2-children in all grades use relatively more words from the first frequency class (the first 1,000 lemmas) than their L1-peers. The effect sizes, however, with eta2 ranging from .09 to .02 between grades, and from eta2 = .01 to nonsignificant between L1/L2, were lower than those of the standardized productive and receptive vocabulary tests (eta2 = .26-.35 resp. between grades, eta2 = .34-.23 resp. between L1/L2), and also lower than the effect sizes for the number of different types in the texts (eta2 = .23 between grades, and .01 between NT1/NT2). The TTR shows only a significant difference in the wrong direction (grade 6 outperforming grade 5). The frequency class based lexical measure MLR discriminates significantly both between the grades and between L1/L2, but the effect sizes are low (eta2 = .05 between grades, and eta2 = .02 between L1/L2). These outcomes show evidence that a frequency class based lexical measure as the MLR is more valid than a type/token based measure such as the TTR.