Intensive Care Unit physician Greg Schmidt, MD, from University of Iowa Health Care tests whether face masks impede your breathing or make you retain carbon dioxide.
"Is it possible to get Legionnaires’ disease from wearing a face mask or wearing a cloth face covering that hasn’t been washed?
You cannot contract Legionnaires’ disease from wearing face masks. Legionella bacteria is transmitted by aspirating drinking water or breathing in water droplets. Legionella is not spread from person-to-person in respiratory droplets nor does the bacteria survive on dry surfaces. Your mask would not be a source of transmission for the Legionella bacteria."
From Legionella.org, https://legionella.org/faqs/general-public/
The site is maintained by researchers.
There does not appear to be much scholarly research on children and masks. The following two articles were the most relevant items found during a recent, thorough search.
Most research on masks and limiting the spread of viruses has focused on the mask protecting the person wearing it. COVID-19 has changed that focus. The articles below are listed in chronological order, dating back to 2008.