Hi there, I had a similar issue in April but my baby was in a basinet. I phoned up to make sure my baby had been booked on, he had but the travel agent hadn't booked the basinet. This won't apply to you as 18 months is too big to fit in it.
You will probably be allocated a bulkhead seat anyway (unless there are other babies on board)
Phone up and make sure the infant is included, as for bulkhead seat (they might not give you it or can't promise)
Don't try to reserve the seats online
Try the online check-in the day before but it probably won't work for the infant (book the adults on anyway)
Turn up for check-in as normal and it should all be ok.
Have a read at this
forum/index.php?f=4&t=273473&rb_v=viewtopicand these trip reports will let you know what to expect when travelling with a baby.
forum/index.php?f=6&t=274070&rb_v=viewtopicforum/index.php?f=6&t=274070&rb_v=viewtopicGood luck! y)