I use the Nite Ize 2-6 battery led conversion bulbs. Each of my 2 boys have them. I've tried a number of different size lights and these work the best because the lights are big enough ( I use a 2D and a 2C host ) that they don't get played with as toys. They had a tendency to take their minimags out to the treehouse, on sleepovers, to the couch-fort, etc and they'd get lost. These bulbs will light with almost no juice left in a battery, so when I change batteries out of their toys, I use them for their lights. I also use rechargeable AA cells in adapters. They have an incredible run time even when I feed them half dead cells, and there's no small parts to get eaten or choked on. The advantage to a flashlight over a plug in nightlight is that they can take it with them if they need to go to the bathroom, and they can move it around to point at different shadows. The nightlights always caused some very scary shadows on the wall. I also think they find it empowering to be able to put it on the nightstand, or take it under the covers, or point it at the closet door. These bulbs aren't all that bright, but as a nightlight they're great.
Another option I've used is to put a mini-mag multi-mode LED into a 2 D cell mag. That will run forever and on high be a serviceable light for other use as well. The problem for me is I only have 1 of these set ups and 2 kids. They'd both want that one if I used it.