I feel for you. I hated being a teenager with child guests in the house--no one ever respected my rights, I was expected to "be nice" to the children, and it never mattered if they were brats or damaged my stuff. Argh, bad memories!! lol
And even worse if the mom thinks that "vacation time" means "no discipline" time. I have a feeling her children don't listen well. The two attitudes go together.
Okay. Rules. Definitely no playing with the ducks in the water--watching only. ABSOLUTELY POSITIVELY NO DOGS in the water while the ducks are. That would be disaster.
And can you at least get the ducks up high so if the dogs get in your room they can't reach them easily? That way if it's an "oops" moment it will hopefully be caught before any damage is done.
I really feel for you. I'm a parent and I understand about making the rules and being in charge because I pay the bills and all that but... kids have rights too, and it sucks to be in a powerless situation like that.
The good news is that 8-year-olds (most) are usually fairly reasonable human beings. They are also capable of being responsible. Appeal to their better side and ask them to help you keep the ducks safe. If they feel like they have a job to do in keeping them safe, they're more likely to be a help rather than a hazard. Put them in charge of checking the rules before entering the room, make them the police for the pond rules, etc. You could even let them take turns being the "door monitor" and making sure the door is properly closed at all times.
Good luck.