When Harry Potter entered his Gryffindor dormitory room for the first time he saw five poster beds, each with deep red, velvet curtains. Harry and his roommate keep their curtains open during the day, but close them at bedtime for privacy.
There are many ways you can replicate this look in your child's room, though I doubt most boys would want the red velvet curtains found in Gryffindor dorm rooms. Here is an example of how one family dealt with having multiple children sleeping in the same room. They solved the space issue by creating two sets of bunk beds, all of which have bed privacy curtains.
The curtains seen here are hung on rods that are attached to the bunk beds. Another easy way to add privacy curtains is to buy a poster bed, or a bed with a metal frame specifically meant for hanging curtains.