We visited Paris for the first time as a family of four (kids were 9 and 11). Justine's place was absolutely lovely. Our children loved the bunk bed room, which was on the opposite end of the flat from the main bedroom. The adults appreciated the great and unique light fixtures, the beautiful ceilings, and the the location was superb.
The list of nearby places (bakeries, restaurants, groceries, etc) that Justine provided was very helpful. The Luxembourg Gardens nearby are a short five minute walk with a TON to do. We visited several times and still didn't explore everything the park had to offer. If you're open to walking, many great attractions are within about 30 minutes walk or even shorter metro ride (which was easy to use). We walked to things like the Eiffel Tower, The Catacombs, Cathedral of Notre Dame, the Louvre, the Pantheon, and a bunch of stops in-between.
Everything in the flat was as expected or better, and it was well-appointed. No real advice or words of caution on that front - save for that the address is a "BIS" address, which just means its a few doors down from another entrance with the same number. :)