That isn't true. Masks help, but only under certain conditions, ans there is a trade off. The impact of mask wearing for children imho is higher than not wearing masks.
https://english.elpais.com/society/...-coronavirus-transmission-says-new-study.html
The Spanish researchers found that there were no significant differences in transmission between the over-sixes, who must wear face-coverings, and the under-sixes, who are exempt from the rule
This is not to say that masks have not worked, adds Quique Bassat, a pediatrician and epidemiologist from the ISGlobal health institute. “Masks offer protection, but in young children, aged from three to 11, where transmission is lower and risk attitudes are different to those of teenagers, the impact of this measure is more modest.”