My son (6) and I stayed in the apartment for 5 nights. Everything was well taken care for and we had great communication with Julien and Karima about check-in and during our stay. They also gave us a few tips to see throughout our stay.
The apartment is cool, even in the heat we had during our stay, that was a welcome change from the heat outside.
From Gard du Nord you can take line four to chateau rouge or marcadet poissonniers. Line four that leaves from the latter station will also take you right into town near the Notre Dame at stop Cite. Or you could take linke 12 to concorde. You can walk up to Sacre Coeur within 15 minutes.
I would definitely recommend!