How does a depressed mom impact her child's emotional development?

Severely. The child's emotional development depends on a give-and-take with the primary caregiver: infants learn to replicate the caregiver's nonverbal mood cues, and caregiver's displays of pleasure reinforce behaviors of the infant that led to these positive reactions. A depressed parent is unable to provide these reinforcements.
Delays it. Infants of depressed mothers show differences in their efforts to get their moms to smile and "coo" back at them as young as two months of age. If a typically engaged, interactive mom keeps her face expressionless ( " still face"), her baby keeps trying to get her attention, while a depressed mom's infant gives up very quickly and turns his face away.