"Being deprived of seeing half of the face could be overcome precisely because of that brain plasticity. Babies and young children are far more adaptable to their changing conditions in the world than we are as adults."

Said David Lewkowicz, who studies speech and language development in young children, quoted in "Do masks for young children impede their language development? Research is sparse on this issue. But the few studies that do exist suggest masks do not inhibit kids from learning how to communicate" (WaPo).

There's an understandable effort to assure parents that there children are not being damaged by masks, but in amongst the reassurance, I'm reading:

Research has shown babies who pay attention to people’s mouths as they talk have better language skills when they’re older. Plus, we know babies learn by modeling their caregivers’ behaviors, such as smiling, laughing and talking... 

And anecdotes like:

Emily Langworthy’s 2-year-old daughter, Rosalyn, also attends day care with masked caregivers, and was experiencing language delays. When Rosalyn and her parents contracted the coronavirus and had to isolate for two weeks in January, Rosalyn’s language exploded, Langworthy says. She believes it’s because her daughter was home with her unmasked parents all day.

And advice like:

Most babies and toddlers interact with their siblings, parents and other unmasked caregivers. If children attend day care with masked providers, getting face-to-face, unmasked interaction with their family members before and after will probably offset being around masked adults all day, says David Lewkowicz, a senior scientist at Haskins Laboratories.