@Parentslife
Are they coming to yours or are you going to theirs?
Because all the way through the thread you describe it as though they are coming to yours and the 15 year old is eating all your food and hanging around and your son goes to bed at 7.30-8.30?
Then all of a sudden in this post, hidden pages into the thread, you say "we always go to theirs" and "it's genuinely our turn to have them over"
So basically the whole thing is nonsense. You have been going to theirs. In which case, YABU because presumably (from the way you have phrased it) you are taking the 7 year old to theirs. You can't then say they can't bring their daughter to yours. The girl is eating food in her own home. It's up to her parents to decide whether to tell/ask her to go into another room and play on phone/watch film etc.
If they haven't been to yours for a while, maybe they won't bring her this time. Maybe she won't hog the food in your home (and anyway, surely that's very easily dealt with "Excuse me, Sarah, could you please pop the plate of veg back in the middle so everyone can help themselves? Thanks!"
However, I do agree that it is annoying to have a 15 year old hanging around and yes, there are plenty of topics you can't discuss in front of them (despite what people on here are saying). It's not just about shagging and anal beads and who knows what - but you often want to discuss worries about finances, the world, jobs, health etc.
We have a similar problem at the moment with a woman who brings her 17 year old to a hobby group. We have one hour each direction in the car where we are only allowed to talk about the 17 year old because the mother constantly directs the conversation. At the group she hangs around with us and the mother complains if the talk gets onto any adult topics such as when one of the group needed a shoulder operation and was talking about it. That wasn't suitable for the 17 year old's ears for example. It's difficult and hard to shake them off. There are other young people there she could talk to, but she doesn't. So I do get where you are coming from regarding that.
There's nothing you can do about it if you are going to their house. If they are coming to your house for the first time in a while (totally unclear based on what you have posted), you can set up some new ground rules if you like - such as plating up food, or just telling her to put the food back in the middle, or "leave some for someone else please, if there's any left when everyone has helped themselves you are welcome to have it" and then after the meal send her off to the living room - TV etc.