Hi Marilyn:
Good friend and fellow moms panelist,
Erin F, provided a great answer that I'll paste below and link to
here.
Enjoy your vacation.
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The laws for child car restraint vary in the US from state to state. For an overview of the situation, you can go to the website statehighwaysafety.org and search for an article called "child passenger safety laws." Some states do have laws based on the child's height or weight, but in Florida, the law is by age. If do you need a car seat or booster seat for your daughter, you will be able to rent one from all the major US car rental agencies. Also, many of the taxis in the Orlando area have car seats available. If you are at the taxi stand at the airport or if you are getting a taxi at your WDW resort, you can always request a taxi equipped with a car seat.
For transportation on the internal WDW system, you will not need (nor would you be able to use) a car seat. The WDW buses, monorails, and boats do not have seatbelts so you would not be able to secure a car seat to them.