No way. Far too far and far too young!
YANBU!!!
This is the kind of thing that would be about 100 times more fun for both them AND YOUR PIL if they just waited a few years.
If something happened, you couldn't get there at all easily. Yes, it's unlikely that anything major is going to happen. But what is almost guaranteed is that after a couple of days they're going to want you there. So they're going to be tired, emotional, and wanting mummy, and you aren't going to be able to be there. Im afraid that at 5 and 6, it's more likely that the 'memories' are going to be of the slightly scary time they had with Gran and Grandad and Auntie X all trying to jolly them along at bedtime/when they fall/when they argue and cry when they both just want you.
At 5 and 6, so much of the daily care is still quite tiring and time-consuming, too - and very tailored to THEM in particular. You know, for example, that perhaps the 5 year old will enjoy doing x as long as they've had an early night the night before, and to change plans to get the best out of a particular day if they've had a bad night's sleep. That's just one example: eating, sleeping, resting, favourite foods/toys/colours, flash points when they argue - EVERYTHING about them, which you know and your PIL don't. It's stuff like that that really matters when little kids are away from home and expected to 'keep up' with a busy schedule. Holidays may be fun, but they're also busy, tiring and always on the go. Your PIL might be in for a rather disillusioning shock if they end up with two cranky, squabbling children who quite frankly aren't going to appreciate the fact they're in Disneyland at a cost of ££££££ when neither of them had what they wanted for breakfast and are tired and want to go home now, please. Basically, from your PIL's point of view, this could end up one fantastic way for them to waste a shedload of money having a rather stressful time straining their relationship with their grandchildren. That really needs to be pointed out and the rose-tinted specs taken off for a bit.
And that's without even getting to the fact that yes, you're the parents, if you would rather do these big holidays WITH them, then you should.
All this could be avoided if they waited a few years - when they were more independent, better able to manage tiredness and being away from you, PLUS - they'd actually be old enough to properly remember it!
So I would absolutely say NO to Florida. Butlins, yes. Florida? Not until youngest is at least 8.