OP - just state your case, and then you can stop worrying about it - hopefully you have misread the situation, and they might not be assuming anything of the sort.
I agree with others, though, if there is an assumption, please don't heap all this on your DIL, it's unlikely that she has single handedly come up with the assumption without some input from your DS.
Enjoy your retirement as you please - being able to do things on the spur of the moment is surely one of the joys of not being tied down with work.
Maybe you could offer to cover the childcare a couple of days per week during school holidays? You won't be wanting to jet off then, presumably, because of rise in prices, and everywhere swarming with school age children.
This is what I think I'll do when the time comes. I get one day off per week, but I will need that day to do housework, catch up on errands etc. However, I work in a school, and so am off every school holiday, that's when I'll be worth my weight in gold!
. It'll be much more fun being able to take them for days out, rather than be tied to a routine, collecting tired and grumpy children up from school and keeping them amused till they get picked up by tired and stressed out parents
.