On a weekday morning at ABC Little Steps Daycare in Harlem, the infant room is full of coos and giggles. Amid the bustle, one teacher’s assistant stands out with her calm, steady presence. For more than a decade, Mei-Feng Cheung has been a cornerstone of the daycare, feeding infants, rocking them to sleep, and earning the trust of families who often invite her to babysit outside of work.