The word I've always heard for "night-light" (or night light or nightlight) in French is une veilleuse. The French verb veiller [vay-yay] means to sit up (with someone who's sick, for example) or to stay up late into the night with company. In other words, it means to choose not to go to bed and sleep. I guess a veilleuse [vay-yeuz], a night-light, is a lamp that stays awake, in its own way, to protect you. Protect you from bumping into walls or furniture, or falling down the stairs, when you're up and moving around in the dark.
Here are a couple of photos of the stairs that lead up to our loft space from the main living area (kitchen, living/dining room, bathroom, etc.). They show the new veilleuses we bought and installed a few days ago. They are light- and motion-sensitive.
By the way, I don't know how we would live out here in the French countryside without on-line or mail-order shopping services like Amazon.fr and others. To find and buy items like these night-lights in a store, we'd have to spend hours in Tours or Blois, the two closest cities, walking or driving from place to place searching for them. It would be a two-hour round trip to go shopping, and much of that time would be wasted if we didn't happen to find the items we were looking for. Amazon.fr also has a very good return policy, in case the items purchased don't fit the bill.
This second picture might give you an idea how steep these stairs are. They are also slippery, and maybe we need to do something about that. Anyway, the night-lights help a lot. They come on when the house is dark and when something moves within range of their motion detectors. When there's plenty of ambient light, whether from other lamps or just from daylight, the night-lights don't switch on when they detect motion. I think they stay on for about 30 seconds, which gives me plenty of time to get down the stairs without breaking my neck.
I've been surprised to see the term lampe de nuit for night-light on Amazon.fr. I guess it's just a word for word translation of the English-language term. Veilleuse is also the French word for the "parking lights" ("sidelights" in the British Isles) on your car, and for the "pilot light" on your gas stove, furnace, or water heater, for example.