A photo-resistor circuit

Hi

I am interested in building a simple circuit with my kid. I want to connect a battery to the circuit. the purpose is very simple - a LED light should turn on when the room is dark (we are building a light-night for him ).

How can I go about doing this? is there any link to the internet explaining how? or maybe someone here? I though of using a photo-resistor but i am no expert, so any tip would really help.

thanks alot.