A few ideas on a very important issue.

TIP : Change a light bulb to a florescent one or plant a tree with your child.  At the same time simply explain the good it will do the environment.  Even at the age of 4 and up kids will remember and it will have an impact

TIP Get a solar kit, such as a solar powered car, and put it together with your child.  It an great illustration of how things can work without electricity.

TIP: Power Standby. Many household appliances have a power stand-by when they are using power unnecessarily.  These include computer, microwave, TV, stereos etc. 

The common advice is to turn the appliance off at the wall.  But this is not always the easiest thing to do.  Heres 2 suggestions...

Remote Off:  there are devices that plug into the power outlet that can be use to remotely turn off an appliance at the wall.  See Dick Smiths

Timer Off: timers can also be used to automatically turn appliances off at night when everyone's asleep and have then turn on again in the morning.  The digital timers with a battery back up  are best and cost around $20 from Coles