We spent a while looking into this for our trip at the end of April, a happy 4 hours on the phone in fact...
If you book platinum insurance via Virgin Holidays then you are covered for seats - but beware, booking platinum insurance of you hire the car through
Virgin Atlantic (as we did) they aren't - despite the car hire being provided by to VA by VH! To hire the seats was going to cost £10 per day per seat, which was $280...just shy of doubling the hire cost for the car.
The law is different, a nice lady I spoke to at Virgin Flying Club sent me a link to a really useful page on the NHTSA website but I can't find it any more, and I cant post a link to their homepage yet. :| So, we found out our 4yo will just need a booster, as he's tall for his age anyway. This would have made hiring a little cheaper.
I checked with the Flying Club people that Virgin Atlantic carry car seats for free, but I was concerned about them getting damaged at Orlando International (they kindly wrote off a samsonite case for me between the plane and the terminal a couple of years back) so I've borrowed an old spare seat and booster from the in-laws. That way we can keep our recaro seats damage free!
In the end, since I had booked pretty much everything with different people I ended up hiring the car through someone else as well... £340 for a fortnight in a full size with all the insurance. Seemed OK to me...
Si.