More POB than seats?
Two different things, as 212 already pointed out. Depending on local legislation, infants may typically be carried on the lap of another pax but need to be properly secured using a child restraint device. So yes, in that scenario you might have more POB than seats.
All other children (older than infants as defined by the law) need to have their own seat.
MGW and performance restrictions always take precedence.