I agree. After doing some Doom Googling, I'm
concerned about CO2 levels when children wear face masks. Most articles I've read give reassurance that the increase in levels aren't that significant, but to my mind this doesn't jive with studies showing that
merely living "downwind" of a highway can decrease a student's test scores or that
breathing in phytoncides (being around plants, essentially) can impact humans fairly substantially. It seems to me that humans are extremely sensitive to air quality in other situations that have been studied, I just don't see how upping carbon dioxide levels, even a small amount, for hours and hours a day can be casually dismissed as clearly having no effect.
On the other side, the risk of unknown side effects from Covid (like, as mentioned upthread, diabetes,) are of course horrible for parents to think about.
If there were a fairly clear consensus on the risks and benefits of masks for young children it would be different - right now I do think it really is the wild west when it comes to information on the topic. Everyone is winging it, to some degree, and parents have to do the best they can with available information.