You can book anything that your child needs for free on most flights, actually virgin and jetstar definately do it from experience! You can book child seats, strollers, prams, port-a-cots, childrens luggage and all. They will take it, I've flown with both carriers you're going with and taken on a lot of stuff for the girls and they havent worried about it. Also you ask if you can keep the pram until the last minute, which is normally a couple of minutes before you board. Whcih is also handy.