Many llc's these days work on employing the absolute minimum number of F/A's that they can get away with, DA - using the case of the popular B737, this is usually 3 F/A's for a 150 seat configuration.
However, it is not unusual to see a full load of 150, PLUS (say) 6, or 8 infants - a total PAX count of over 150, which would -employing the formula of 1 C.A. per 50 pax (or unit thereof) incur the NECESSITY to have another Cabin Attendant on board, for Safety reasons.
You are correct, DA one of the roles of an F/A is "to open the doors and scream at the pax to get out", however another role, following an evacuation of an aircraft is to check the cabin to ensure that everybody has vacated.
There is no guarantee that the person(s) who carried the infant)s) on board is going to survive, and to demonstrate this necessity, ALL of the airlines with whom I have been employed have small, life size dolls that the checkers will place on/under seats of the mock-ups used during aircrew emergency training and re-validations, to make sure that the infants are all accounted for.
After all, infants ARE passengers........aren't they?