trickiewoo - our eldest child was born in new york (north shore hospital in manhasset - we lived in great neck) after we had been living there 3 years. that was 19 years ago though and communications were not what they are now - we had to make do with letters and occasional (very expensive!) phone calls - you are lucky!
her birth precipitated our return - when she was 15 months - not having NHS support, or family, made me, at least, feel very vulnerable. personal security was also a factor - there are a lot of guns in america - many of them in the hands of people who are less than stable.
floridamum, schooling is also a major factor to consider. i chat on a games site to a mother of 4 in north carolina and she is home educating her eldest (aged 8) because she is not happy with the state (public!) schools there. (her others are all pre-school).
they don't start grade 1 in the US until after they are 6 - prior to that they are in kindergarten - your girls will probably have to be put in with older children - could they cope?
(a girl of 15 (Y10) at my daughter's school moved to america earlier this year, and has just returned because her educational level meant she needed to go in a senior high school class with 18 year olds and she didn't want to.)
it's not all negative, we did have a brilliant time there for 4 years and we lived in an expensive place with cold winters. housing in florida is quite cheap and it's so much warmer than here! (and in orlando i can imagine you will be turning visitors away for most of the year, they will be lining up to see you!!!)
trickiewoo's suggestion of a computer for nanny and a webcam is a great idea, although saying goodbye each time might be hard...
it's not a decision to take lightly and your daughters can't really imagine how it would be to live in another country, so the decision is firmly in your hands; if your prospects are really good and you are basically very keen to go yourselves you will have to talk the place up a lot and talk down the distance, etc.
and while leaving your cats would be another major wrench (could nanny possibly take them on if they can't come? or a friend? so you would still be in touch? i didn't think there was quarantine going into the US, but if there is it would be a lot for elderly cats to cope with) maybe once you were settled you could consider getting a kitten for them...
when we first went to the US we did it quite blithely, having rented accommodation here and no kids, but when we got there (minnesota, not new york initially) all the other brits who had come out with the same company had kids and mortgages and they all managed pretty well - the oldest kids we knew were younger than yours but some of them were school age; and one of the families is still there, 2 kids married, one in the army and first grandchild born last year! (they live in kentucky now.)
is the job with an american company? will they give you lots of support and guidance in choosing where to live, setting up home, finding schools, establishing a credit record (UNBELIEVABLY important!!!), opening bank accounts, making friends etc? if they will that makes such a difference...
since you first posted 4 weeks ago i do hope all this is not too late!