Yes, if there's any chance you can afford the airfare, get a seat for your child and bring her/his car seat. This is a long flight to HI even from the West Coast and everyone will be happier if your little one has his own seat/car seat.
When my son was a toddler we cruised Hawaii and rented cars in a few ports. It's a very affordable and easy way to go w/kids. We loved driving around Waimea Canyon.
Luisa Frey Gaynor
Family Cruise Editor