From Fl state website: For children aged through 3 years, [a] restraint device must be a separate carrier or a vehicle manufacturer's integrated child seat. For children aged 4 through 5 years, a separate carrier, an integrated child seat, or a seat belt may be used.
I have a 6yo who is not required to be in a booster in FL but is required to be in one in our home state. To avoid confusing this child we are bringing his booster with us. We will just check it with our luggage. Each member of our group is allowed 2 checked bags, we will have less than 2 per. so we will be counting the carseat as one bag. Just tape name to it. Some airlines even let you check one and not count it against your total. It doesn't hurt to ask.