Body in Bed
Create a "body" in your own or the child's bed using pillows, a rolled up blanket, or other appropriately shaped objects. At the top of the body, put a scary mask, with a wig if possible, resting on the pillow looking up at them. Pull the covers up over the face until just the hair is sticking out.
If you place this in the child's room, when he goes to get into bed in the dark --- or if he enters the room and turns the light on --- he will definitely jump. If you place it in your room, and the child enters expecting to find you there, he will also panic when he discovers the scary mask staring up at him.
Head in the Closet
Many kids are scared of the closet, particularly at night when that isolated part of the room is almost always dark and shadowy. Play off this fear by hanging a scary head or ghoulish mask from the bar in the closet, right at the child's eye level. When she opens the closet, she will see a creepy face staring out at her. Bonus points if the child is tall enough that you can hang the head among the clothes, so she will unexpectedly uncover it while rummaging through them.
Hand in the Dark
Kids will easily be scared when an unexpected hand comes at them in the dark. You can use your own hand or a skeleton, monster, or otherwise mauled hand from a costume or party store. For a supremely frightening effect, crawl under the kid's bed before he come in for the night, then slowly push the hand up over the edge of the bed and under the covers. The child will scream when he realizes a strange hand is grabbing him from under the bed. You can also enact the same move over the edge of the couch when the child is engrossed in watching a movie in the dark.
Voice in the Night
An unexpected and unknown voice in the dark will send chills down a child's spine. Hide somewhere where you know the child will be, like her bedroom or any other dark room in the house. Wait until they enter the room, then say a spooky phrase like "I'm watching you" or "I know what you did" in the deepest voice you can muster. Other words guaranteed to frighten are the child's name, spoken in low tones, or meaningless phrases that invoke a haunted house vibe, such as "Look out for the butler" or "It was Jack." For extreme jump factor, scream suddenly, or shout in a horrified voice something like "Help me!" or "Don't hurt me!"