In a futile attempt to actually answer the OP instead of bi#*%ing about kids travelling....

Typically, aircraft have one more oxygen mask that drop down than the number of seats in the row. For that reason, if you have 2 lap children, mom and dad must sit in different rows, or across-rows, with one child each.

As an example of an exception, from iFly - "Lap-held infants traveling on our 50-passenger jets must sit on the right-hand side of the aircraft (seats C or D) in any row other than an Emergency Exit Row, since an extra oxygen mask per row is located only on this side of the plane. Please ensure you select seats on the right-hand side of the plane if you are planning on traveling with an infant on your lap. On the Airbus flights, lap-held infants can sit on either side of the plane."