Thanks for the recent positive posts! ParisAmsterdam - I think you are right, the parents attitude is probably one of the most important things.
Our flights are now booked... we are spending a week with family in Washington DC area, then we will fly down to Florida and spend one week in Key West on our own, and then be joined by family for a week on the beach in Captiva. I'm happy with the plan we finally decided on. In the end it was the multi time zone changes that made me reconsider our Hawaii plan... now all in the same time zone is one less complication. Hawaii will wait for the future!
I first went to Hawaii with my parents when I was 15, they also had a 13 yr old and a 5 yr old with them. They look back now and can't think how they did it, but we had a great time!

Your original plan is no more overly ambitious than when we took our 2, 4 and 6 year old to Spain for a holiday. We've been traveling with our kids since our oldest (now 16) was 11 months for a family trip to London.

You can do it. Children are resilient and travel well.

The keys are fine too, but I do think you could do Hawaii.

Have fun!

Here's a tip - get one of those little DVD players and bring some videos.

Florida is soooo much closer than Hawaii.

Check out VacationRentals.com for home rentals - there are tons that would work well for a family with a toddler.