Only read the first 5 pages, so sorry if this has been said subsequently…..
10 months time puts you at June/July next year. That’s in hurricane season for Florida. Personally, I wouldn’t book an expensive trip like that to fall in the most dangerous time I could go - either your trip could be ruined because of a hurricane, or you could get caught up in one! Admittedly it’s early in the season, if you are looking at June, but it’s still a risk.
Another thing to consider is do you ever want to take your children there too? If so, maybe put the money aside instead to go when they’re older, because a change in financial circumstances could mean you’re never able to afford it again, and you might regret that.
Personally, I wouldn’t want to leave my kids for that long, especially that young and travelling so far away. You mention you work full-time so you hardly see them anyway - doesn’t that make you want to go even less? I hated the fact I saw so little of my children when they were toddlers/pre-school age due to working.
Other things to consider, if you breastfeed, you may still be breastfeeding at 10 months; the baby may still be waking multiple times a night (my first was sleeping through at 6 months - my second was over 3 when he finally slept through); going from one to two babies is a lot more than double the work, so maybe wait until you have the baby here to see if your in-laws are still ok with the idea; you say the in-laws are early 60’s but fit & healthy - my extremely fit and healthy Grandad had a massive heart attack just into his 60’s with no warning at all so don’t assume that fit & healthy now means the same will be true in 10 months time; either child might experience separation anxiety - or both might - and you’d feel awful leaving them; the time difference will make it tricky to call and speak to them, and you calling might upset them so you may have to go the whole time without calling them; you might develop PND and just not want to go on holiday, or leave your children behind; if your in-laws struggle the whole time they may never offer to have the kids again, even for one night; if there’s an emergency, could you get home in time?
Could you look at alternatives? Closer locations and maybe a shorter break? Or, assuming you live in the UK, what about a UK theme park tour? That way you could be home the same day if something happened, but still get your thrills! Disneyland Paris was suggested a few times upthread too which is close enough to get home from if needed.
Finally, ask yourself this - when your kids are grown up, will you look back on their childhood and regret not taking more holidays without them, or will you regret not having more time with them whilst they were little more?
Good luck for the birth, and for your decision!