Nightlight is the light source you should consider. It usually consumes a few watts and is rather dim, but good enough for your case. Some come with photo sensor, so they will switch on by themselves when the surrounding gets dark, and thus save the trouble of turning on/off. Do consider those LED version. I believe the lifespan is longer.

Running on DC is not advisable, especially when the cable is long. Furthermore you have to charge your batteries to be green (not saying charging battery is bad, just that I believe in less battery = higher sustainability). If your mom continues to off the lights, a suggestion from me is to conceal the switch, or an easy re-laying of AC cable for a more permanent solution with the switch in your room.