Use light dimmers. They're great for gradually getting your toddler ready for bedtime -- and you'll use less electricity as you make the room darker.
Replace all your lightbulbs with compact fluorescent lightbulbs (CFLs) -- they use two-thirds less energy and last ten times longer than incandescents, but they're only a buck or two more each.
Buy dimmable CFLs. Not all CFLs are compatible with dimmers, so check before you buy (GE makes 'em for $10.50 each).