Hi I have a 2 year old and want take her with on a cruise. Should I pay for her when i book, or when i get to the ship
You can't show up at the pier with people not listed on your booking, even if it is a child.
You have to list everyone going on the cruise when you make the booking, pay a deposit for them, and then make the final payment for the entire booking well before the cruise starts. So the simple answer to your question is: you have to pay for her when you book the cruise.