To help your child find a light switch in a darkened room, teach therm to do it with eyes closed or wearing a blindfold.
Playing blindfold games in their bed room and hep to teach them to sleep in the dark. This is why blindfold games with soft toys are a good idea. For example, if your child wakes up crying because they can’t find their teddy, during the day on their bed and place teddy on and in the bed somewhere and have your child find teddy just by feel.
If your child has fears about noises in the dark, blindfold them during the day and have them identify different noises.