Nadia's place is so great for families visiting Paris. Can't stress how convenient the location is- a one block walk to the Republique Metro station, which is a hub of 5 lines. It is also a great neighborhood with tons of local amenities and sights, like the Canal St-Martin. Great place to stay in Paris to be transit/walking accessible to all the "standard" sights in the center of the city, but just far enough out to feel a bit outside of that hustle and bustle.
The apartment is really incredible, especially for families with kids. Yes, it is a walk up 4 flights of stairs. But once you up there, it is a really big by Parisian standards, with really nicely renovated kitchen/bath, comfortable beds and great cross-ventilation with windows on all sides. Because the upstairs kids rooms are up a very narrow circular staircase, it probably wouldn't be appropriate for kids under 5, but for those older, it is great. Great style and design as well.