Child seats are an absolute essential for those travelling to America with children. They are first and foremost required to protect the young persons safety but also to comply with legislation.
A lot of people take their own from the U.K. as they are already utilising them for transport to and from the airport.
There is little doubt the advantages from taking ones own seat(s) are twofold.
1. The cost factor as most airlines transport them free of charge and not as part of the luggage allowance which saves having to rent or buy one upon arrival.
2. The other factor is the familiarity with fitting ones own seat especially after a long tiring flight as opposed to being in a dimly lit garage and struggling with the multifarious fittings on a strange seat.

Bear in mind the Rental Staff will show you the seats and help you pick one that is suitable for the childs age but they will not fit it for you as they would become vicariously liable should any issue occur.

The important thing to remember is that legislation is different in America than it is in the U.K. The following is an extract from the 2016 Florida Staute as it relates to motor vehicles. It is complicated but the relevant points have been highlighted for ease of reference.

Every operator of a motor vehicle as defined in this section, 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 child booster seat may be used. However, the requirement to use a child restraint device under this subparagraph does not apply when a safety belt is used as required in s. 316.614(4)(a) and the child:
a. Is being transported gratuitously by an operator who is not a member of the child’s immediate family;
b. Is being transported in a medical emergency situation involving the child; or
c. Has a medical condition that necessitates an exception as evidenced by appropriate documentation from a health care professional.

The child restraint requirements imposed by this section do not apply to a chauffeur-driven taxi, limousine, sedan, van, bus, motor coach, or other passenger vehicle if the operator and the motor vehicle are hired and used for the transportation of persons for compensation. It is the obligation and responsibility of the parent, guardian, or other person responsible for a child’s welfare as defined in s. 39.01 to comply with the requirements of this section.

The salient points therefore are.
The Parent/Guardian is responsible at all times for the safety of their child
A child seat is not required in a bus or a taxi - But again the Parent/Guardian remains at all times responsible for their childs safety. That means should an accident or issue occur and the child was injured then the said Parent/Guardian can be prosecuted.

Certain Companies who provide vehicles for hire e.g. Taxis and Uber now have the facility available whereby child seats can be requested at time of booking. For transportation companies contact should be made before committing to a firm booking