I'm really torn for you on this one! I posted earlier saying that I think you should go for it, and Florida makes me think even more that you should in a way, but it is very far from the ideal way to either have the holiday or start/go back to school.
But it is highly likely to take your child longer to settle if they start later than everyone else, there is no point trying to tell you it won't make a difference. It will make a difference, probably even more so on the back of an exciting and tiring holiday, but the question is whether you think it's a price that is worth paying.
I would be just as worried about your older ds, if not more so tbh. By Y4 they are expected to work, and there will be a lot of stuff that he misses in the first four days. You can't expect him to catch up properly without extra help from his teacher, and I don't think it's fair to expect that of teachers.
At my school our head tells parents not to ask for help catching up when they have had absence for holiday.
It will have a negative effect on both of your children, and I generally think that parents should devote their children's first few weeks at school to letting them settle in. It's a lot for them, and they do need a bit of time and space to deal with it if they are going to settle in as well as possible, and be rested before hand.
When you say you can't afford the £800, does that mean you wouldn't go at all if you can't take those four days?