Sweatingcobbles I think you have two options.
Tell your parents they have screwed up, in the nicest possible way, let them go without you and possibly never invite you in anoter holiday again, or... go... and make the best of it.
Your main concerns seem to be that precious antiques will be broken or your boys bored or other guests disturbed.
Antiques
I think you can go some way towards combating this by:
Asking for any antiques to be removed from the bedroom/s you will use
Making sure your boys know to keep all horseplay etc to the grounds
Boredom
Take lots of things to amuse the boys, comics or annuals, or books if they are big readers, any electronic device that doesn't need the internet (we have a hudl you can download programmes onto, it is very good), take anything else like craft items for use outdoors. Check out what is available in the local area, are your parents driving you, might they be willing to take you to the beach or into town daily? Take balls, frisbys, any sporting stuff to be used well away from the house,. If the boys are into amateur film making take an old digital camera or video camera, get them to document the holiday and find fun things to record.
Is there a DVD player there? Can you afford a cheap one to take?
SYLVANIA SDVD6672UK DVD Player
Lastly, the hotel allows children, your parents have paid to take their grandchildren on holiday. The kids, and you, have as much right to be there are as anyone else. Please do not worry about the other residents too much. If they really wanted to be in a hotel that does not have kids they would have booked one that does not allow kids, they did not!
Just try and relax, enjoy all you can, take bribes (comics, small bags of sweets, bars of chocolate, new items like a DVD or new outdoor game) and ensure boys know the treat is before bed, and they only get it daily if they behave in the day.
Yes, I know it is not that simple, my dd has autistic tendencies, and neither of my children bribe well, but sometimes it works!
Please do not spend the week apologizing for your children.