That is actually a quote from the Sewers chapter of Les Mis, so you'll have to take the question of Paris's gender up with Victor Hugo. :P (I am nowhere near confident enough in French to do a French title in my own words. So I went with a lot of pretentious quotes for titles instead.)

Part of it being a parody of E/R modern AUs is that it's set in the US for no particular reason, and I've already toured the Chicago waterworks museum for it, but honesty, I never need a reason to learn about the Paris sewers.

They were on a mission with a full agent who had WS along for emergency muscle and Natashenka along for camouflage, but then the handler got shot and things went very wrong and Little Natasha and WS have to make it all the way back across Russia without being noticed, with Natasha pretending to a child and WS pretending to be her guardian! Luckily they are both much better at their jobs than anyone realized. The whole thing is in Little Natasha's POV which is both very fun and very hard to write, and the idea was to leave it extremely ambiguous to what extent WS is a) remembering Bucky and his sisters and b) actually trying to get Natasha out of the game instead of getting her back to the Red Room. The phrase "love is for children" appears several times in very different contexts from the way Natasha used it in the movie.