Please turn right at the end of rue du Parc de Montsouris, keep walking uphill and we'll see a couple more hidden dead-end streets to your right.
The first one is called Villa du Parc de Montsouris and we cannot go in as it is blocked by a fence. The second one is called impasse Nansouty and you can take a peek at is varied architecture, but come back here and keep walking up the main street.
By the way, normally in French "square" refers to a playground and "villa" to a large house, but as you just saw, in Paris, they can also mean a small street, sometimes a dead-end, often from a single real estate developer. I realize it can be a bit confusing, I guess it shows how much Parc Montsouris influenced the street names around here.
Let's get closer to it. Keep walking up the street until you see a pedestrian crossing to your left. I'll meet you there.