Lovely flat in an extremely convenient location in the heart of Marais. Lots of shopping, restaurants nearby. Close to the metro, walking distance to the Seine and convenient to a park for relaxing or kids with energy. Enough space for small family. Host was very responsive. We felt like locals at some of the many restaurants we ate in the late evening hours. The park next to the apt (forget the name) is great place to just sit and enjoy some time while the kids can run around. Alexandre was very responsive. The flat is very well maintained and clean. There is a nice desk in main room to park a laptop for your stay-- keep maps and charge phones etc.
The second story bedroom and internal balcony is very cool- the kids loved being up there and the plants give it a great feel. A very nice touch.
As others have noted, there is a small lift and then some stairs for the last story. We walked the whole thing sometimes, good exercise. Also the washing machine is shared with another unit I think (its in the hall) and so couldnt really leave our clothes there and leave. Not a big deal- just something to note if thats important (the washing machine was also in french, and I was too lazy to try and figure out).
One thing I also noted to Alexandre- only one set of keys. It wasn't a huge deal, but with two adults it would have been nice to each have a set in case we were doing different things.