Not sure if the US CDC interim guidance behind all this has been posted already....if it has, apologies
www.cdc.gov/quarantine/masks/mask-travel-guidance.html
In part..
"Conveyance operators should ensure that any person on the conveyance wears a mask when boarding, disembarking, and for the duration of travel. Depending on the circumstances, conveyances operators should take the following actions:"
....
"monitor the conveyance for any person who does not wear a mask and seek compliance from such person; and
at the earliest opportunity, disembark any person who refuses to comply;"
...."Mask use may be exempted for the following categories of people":
"a child under the age of 2 (masks should NOT be worn by children under the age of 2)"