Young children's language skills have a significant positive impact on their academic success throughout school, especially on reading and writing performance. The spread of COVID-19, which has resulted in lockdowns, fewer learning hours in kindergarten, and distance learning, might have affected children's exposure to learning opportunities. The aim of the current study was to compare language abilities (vocabulary and morphological awareness) of two groups of low SES kindergarteners, 110 children who attended seven kindergartens before the COVID-19 pandemic, and 54 children who attended eight kindergartens during the COVID-19 pandemic. While both groups followed the same educational program with the same teachers, the pre-COVID-19 group studied in kindergarten before the first wave of COVID-19 restrictions, in 2018-2019, and the mid-COVID-19 group, during the pandemic, in 2020. The results showed that the mid-COVID-19 group of children achieved lower scores on morphological awareness (d = 0.46 to d = 1.19, p = 0.006 to p < 0.001), pointing to a negative developmental effect of COVID-19 on young children's language abilities. Educational implications of the results are discussed.