French was by far my son's weakest Common Entrance subject, and he had started getting quite worried about it, so I took them over them to Paris for a week one Easter a few years ago. I too found it hard to identify something suitable over the internet, but this worked fine. The kids had some tuition each morning and we then explored Paris.
We had a great time and I have really fond memories. Paris has a lot of the same things as London, eg parks, a brilliant science museum etc, and so the kids were in their element. As it was not a proper holiday we did things on the cheap, ate lots of crepes, and generally just explored.
In terms of tuition, though different from the summer course, they covered a lot of ground with teachers used to teaching children. Both children gained a lot of confidence and though neither is a natural linguist, French stopped being a problem subject and both can do more than get by in French. Inevitably and rightly there was more emphasis on speaking and understanding that normally happens in a classroom in England and my daughter, who was younger, picked up a good accent. My son just got an A* in his GCSE, and though what he learned in a week will have been limited, I think it was worthwhile and helped move him from the bottom of the class to a comfortable place in the middle.
Pierre et Vacances who have accommodation nearby, are the French equivalent of Centre Parcs, and are fine and family orientated. The summer campus is a college or something during the academic year but taken over by the language school each summer. Aquaboulevard, which is nearby, is a vast waterpark, both indoor and out. Around 15 slides and all sorts of wave machines. My idea of hell, a view which was unfortunately not shared by my children.
We stayed in a nice family run budget hotel, near the school's main location in Central Paris and about 20 minutes from the Gare du Nord. Hotel de la Cite Rougement. Really friendly and all part of the adventure.