Just another thought, I personally would wait until October and take the 4 year old out of school for a week extra to Half Term.
Our school has never minded this as long as we ask permission in advance, and it is such a long first term for the reception children, many of them will be very tired and prone to colds, coughs, tummy upsets, and thus end up missing school anyway! I know there's lots of publicity at the moment about not taking kids out of school, but the penalties relate to unauthorised absences. Many children start doing half-days until half term anyway. (Just my opinion, not wishing to start the whole school absence debate again!)
I agree it is easy to take a newborn away, I took my 4 week old to Marrakech for a long weekend (freebie as my husband was at a conference). Baby was fine, all he needed was me, nappies and a cot! However I would not go to Florida with a tiny baby because of the sun/heat issue, also I would want the baby to have had all injections/ immunisations at 2, 3, 4 months. Also it is a long way to go if your daughter needs to stay out of the sun with the baby, will she be able to enjoy the parks, or experience her daughter enjoying them?
You could always consider a trip to
Disneyland Paris in the meantime!
Anyway, fingers crossed for a successful pregnancy.
Michelle