This was in a post from last year:
Straight from Florida's DOT website:
All children 5 years of age or younger must be properly restrained no matter where they are sitting in the vehicle. Children through age 3 must be secured in a separate carrier or a vehicle manufacturer's integrated child safety seat. For children aged 4 through 5 years, a separate carrier, an integrated child safety seat, or a safety belt may be used. (FL Statute 316.613).
316.613 Child restraint requirements.--
(1)(a) Every operator of a motor vehicle as defined herein, while transporting a child in a motor vehicle operated on the roadways, streets, or highways of this state, shall, if the child is 5 years of age or younger, provide for protection of the child by properly using a crash-tested, federally approved child restraint device. For children aged through 3 years, such 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.
My experience has been that some car rental companies will claim to have loaner kids' car seats - but the seats are not always available (even after "reserving" them) when we arrived.