We (a family of three including a 1-year-old toddler), came over on the Eurostar from London for a winter long weekend. This place is an easy 10- to 15-minute walk from the Eurostar terminal at Gare du Nord; if the metro wasn't on strike, it would be even faster on Line 4. This quiet second-floor studio flat is set back from the street and gets a good amount of light; it stays very warm, but is carefully laid out to feel open and spacious, with a few pieces of versatile furniture that can be moved or packed away to make more space. The kitchen and bathroom seem newly re-tiled and are in good condition; the kitchen has a few pans and utensils for the stove-top, enough to make a simple meal. There is a wine bottle opener (essential), but we could have made use of a bread knife in case we brought a particularly crusty loaf back from the boulangerie. There's a Carrefour supermarket within about a minutes' walk, plus dozens of market stalls and shops nearby.
Antoine managed our stay; he was in touch ahead of time to confirm our arrival and guide us to their place, to meet us and show us around. He kept in contact throughout our stay and made sure we knew he was available if we needed anything. Charlotte sent us suggestions of walks and things to do around Paris. The cot and highchair they provided worked very well, and the open layout of the flat meant our toddler could roam pretty freely; perhaps other toddlers are less destructive than ours, but we did have to move quite a few loud or breakable items from low and open shelves to higher ground, out of reach of tiny/greasy fingers.
The reason you want to stay here is the access to Mont Martre. Start walking uphill and in a few streets you'll be amongst dozens of cafes, bars and restaurants. Near the top of the butte, across from Sacre Coeur, we found Hardware Societe and Soul Kitchen to be very good places for coffee and breakfast that are off the main tourist track: the former is an Australian-style brunch place that can have quiet a long line on weekends; Soul Kitchen does a more traditional petit dejeuner. The location of this place is ideal.