https://www.scientificamerican.com/article/masks-can-be-detrimental-to-babies-speech-and-language-development/

My daughter’s friend was recently alarmed when she was told that her two-year-old must wear a mask in preschool. Her little girl already struggles to make herself understood, and her mother worries that the mask will make it harder for her daughter to be understood and that she will have trouble telling what her masked peers and teachers are saying.

Now that the face mask has become the essential accoutrement of our lives, the COVID pandemic has laid bare our fundamental need to see whole faces. Could it be that babies and young children, who must learn the meaning of the myriad communicative signals normally available in their social partners’ faces, are especially vulnerable to their degradation in partially visible faces?