How to activate a "sleeping" child object on trigger?

Okay, so I’m a Unity rookie and need to know what the script for activating an inactive child object would be. Like, the enemy in my game touches you, and a particle emitter activates “on” you and everything turns black. Any help?

For the particles:
OnTriggerEnter or OnCollisionEnter to test for the collision and then instantiate a particle system (or move a preexisting particle system to the location and start particle emission).

Turning the screen black:
Have a function in the particle effect to set a boolean value true after a set amount of time and a black texture GUI element that is displayed when said boolean is true. Call the function on Start (or, if you use an existing particle effect, via SendMessage).