As general advice for those not used to travel frequently with infants, bassinets normally suit only for babies until 6, maximum 8 months old (8 only when they are not big on average), if older do not waste your money and time in booking bulkhead seats. Even more the fixed armrest turn out to be very uncomfortable when your baby wants to sleep in lap, in the normal seats you can always lift the armrest and infant is able to lay down/sleep between the two parents laps.
Happened to me when my daughter was 9 months old already even she was slightly smaller than average for her age, could not fit the bassinet thus tourned out asking FA to remove the bassinet, luckily at that time I did not pay any surcharge to get bulkhead seats.