I believe the harm of masks greatly outweighs the benefit, but I also believe that we have to have our facts straight if we hope to win people over. So for that reason, I would like to make one correction/contextualization:
I’m not sure about Germany, but there’s a reason OSHA (the U.S. Occupational Safety and Health Administration) regulated environmental carbon-dioxide levels (those in your workplace), but not your real-world exposure. It’s that OSHA is using carbon dioxide as a proxy for proper ventilation, not that it finds a danger at slightly elevated carbon dioxide levels.
If OSHA’s prohibited-levels caused actual dangers, I would be whole-heartedly endorsing everything we could do to immediately stop combustion power generation. I do note that the German report found much higher still levels of carbon dioxide from mask-wearing, but presumably not from the sort of masks required of children.
If I remember correctly, it is the buildup of CO2 that triggers us to exhale. While exhaled air has much more CO2 (35,000 to 50,000 ppm) than inhaled air, wouldn’t an increase from mask usage accelerate the need to exhale? Would doing that for hours each day be unhealthy?