FlightDetent...
You have actually asked a very good question. While I cannot speak to the JAA/EASA operational requirements specifically, I do have some information that you may find useful.
As you have noted, the maximum passenger load is specified in the Type Certificate and has been established as:
2.3.6. Maximum number of passengers - 220.
EASA TCDS A.064 - A318/319/320/321
The question that needs to be answered is whether an in-lap child is considered as a "passenger" or not. During aircraft certification, simulated lap children are carried by passenger participants and are in addition to the number of passenger seats for which the manufacturer is seeking certification.
Here in Canada, we have an advisory circular that provides guidance to operators in establishing their procedures when considering the carriage of infants. The established policy that I believe relates to your question is:
An infant secured in a lap-held position by a parent or guardian passenger is not counted as a passenger for purposes of determining the minimum number of flight attendants required on board an aircraft, and the maximum number of occupants authorized to be on board an aircraft. However, the infant is counted as a passenger for purposes of applying regulatory requirements such as those pertaining to oxygen, life preservers and survival equipment.
Advisory Circular 0116 - Infant Passenger Count
I am not certain whether JAA/EASA has similar guidance published or not.