Both of ours were about 10-months when they first went to Aruba. in our case we didn't purchase a ticket since we figured we be holding them most of the time. At the very least, the extra seat you have will give you some extra room.

In our carry-on we packed a small tin of formula for the flight and larger one in our checked bags for our stay on the island. We also brought a few snacks for them and some toys likely to distract and entertain them.

We packed enough diapers for the week in our checked bags as well. By the end of the week there was plenty of space in the suitcases.

We left the pack-n-play at home. Instead we bought a small pop-up cabana tent for them to play in and stay out of the sun. Neither one of them wanted anything to do with it, but other kids on the beach enjoyed it.

While they bare fit in it anymore, we took them in the carry around type car seat that locks into a base. We gate checked the seat, but if you have one it may be approved by the FAA for use on an aircraft. We left the base at home as our could be used with a seatbelt.