We really enjoyed our stay in Montmartre! Comfortable space, awesome location, well-stocked kitchen. We stayed with a 3-year-old and a 9-month-old, so my specifics about that are:
* great park practically across the street, which both kids enjoyed
* neighborhood is rather hilly (and one side of the hill has a lot of stairs) - can be challenging with a stroller
* the apartment itself is on the ground floor, so easy with stroller - I was so grateful for that!
* many kid-friendly eating spaces nearby
* comfortable bedroom for kids, with a pull-out bed lower to the ground - good for our wiggly 3-year-old
Note that the concierge mainly speaks French (and I think Portuguese?), so communicating with her directly will be harder if you have no French.