We picked up a pack of these cards, by EcoAction Games, in central London and the simple pictures could be useful and appealing for children from about 5 to 8 or 10. Younger kids might not be able to play the game in earnest, but the cards are a great tools for talking about energy and the environment.
update for 2017
EcoAction Games has abandoned the name Eco Action Trumps to avoid association with the incoming US President Donald Trump. For just the price of the postage and processing you can get one of these old decks: