we had an faa approved combination car seat/airline seat/stroller that was perfect for traveling with Sarah when she was that age. I'm sure they still make them. In fact, here's one (expensive, but worth having for travel)

http://www.amazon.com/Lilly-Gold-Str.../dp/B0006UF43M

I recommend getting a seat for your own sanity and your childs safety. Hopefully you are elite and qualify for elite status and you can get 3 seats together in E+ which will be quite comfortable for you and your family.

I'm sure you will be getting a car on both Maui and Kauai, so you will need to bring the car seat anyway. (I'm assuming you don't want to rent one from the car agency and whatever filth, etc comes with it.) With 3 pax (assuming a seat for the baby), you will be able to carry on everything you need for the long trip.

We did the infant/lap baby thing exactly one time. That was all it took to realize that it was worth it getting a seat for the baby.

-David