Family visits are so very important - I grew up overseas and those few-and-far-in-between visits from Grandparents and other relatives are clear, wonderful memories in my childhood. You are absolutely doing the right thing with making that time a priority.
I think calm reassurance is what your daughters need. "of course it'll be a little scary, but then, you will be in Hawaii, Grandma & Grandpa will be so-o-o happy to see you, and you'll never have to worry about being scared of that again! Besides, did you know that flight attendants are told about children who are having a travel adventure? They look out especially for you".
The fear of doing something new and a bit daunting is normal. Let them know that, but remind them about how important it is to get to spend time with their grandparents.("twice as much time as Mom is going to get!")
Homesickness is a normal part of growing up, and they should know it's fine to feel it, and that it soon dissipates. With modern technology, they can call you from the airplane as soon as they touch down (and you should arrange just that, maybe).
~~ and you do want to make sure there are lots of choices for activities on the plane, -- some each of snacks, books, puzzles, cards, dvds or ipods. Write little notes, and sneak them into backpacks or between the pages of books as 'hugs' from you.
I do hope the fear subsides and the idea of adventure creeps in! With gentle reassurance, I bet they'll be fine.
note - we now have my elderly Mom fly "escorted" because it's taken the anxiety out of the equation (for her AND for us), gate to gate. And we can take her to the gate when her ticket is so marked. Please tell them our Grandma Fran is eager to travel again now, and we're all so happy.