It's their house. But don't be afraid to take the kids outside to enjoy the weather (and say you are getting out of their hair for a bit) - I would think that is the point of being there anyway. If you will have a car available, then explore the area. Ask them for suggestions.
But also just relax - vacation is a change of pace anyway. If the TV show is inappropriate, then just say that you don't think your kids will "get" it and suggest another activity. After all, they can watch TV anytime and maybe a game or a walk would be fun.
Keep your kids to a bedtime that keeps them from being cranky or overtired, and allow for trouble sleeping in a new environment, but otherwise don't feel necessarily tied to 8 PM. Once you get home, your rules go into effect again.
If these are really good friends (which I assume they are if you are spending time in their home), then why not discuss your concerns beforehand?