Whether your kids share a bedroom out of choice or necessity, you need to provide them with a bit of privacy within that space. A room divider will accomplish this as well as visually splitting the room into different sections according to ownership, usage or purpose. Many different options exist to divide a room, from semi-permanent structures to dividers that are easy to move and rearrange when the kids needs change.
If the kids share a bunk bed, hang curtains on the lower bunk that can be closed for privacy; hang another set from the ceiling, if desired, that can be closed around the top bunk. If there are separate beds, divide the room by hanging strings or panels of something decorative from the ceiling. You can hang strings of beads or lights from a row of cup hooks. Install a wall-to-wall, ceiling-mounted curtain rod across the room, and hang floor-to-ceiling curtains or long strips of ribbon; both can be pulled aside during the day to open up the room and can be drawn closed for privacy at night.
Running bookcases back to back down the centre of the room will increase the kids' storage space while defining each area. You can also place a line of tall, paired dressers or hutches, staggered so that alternate pieces of furniture face the opposite sides of the room; glue sheets of cork to the backs of the furniture to make notice boards, or let the kids hang posters in the "empty" spaces. You can also buy each child an enclosed loft bed; they sleep at the top of the structure, above a small study or play area with privacy walls.
Decorate wood-and-fabric tri-fold screens for flexible, storable room dividers, or connect three large shutters or wooden doors with hinges, and stand them between the beds. Attach a top-mounted sliding closet door runner on the ceiling, and hang mirrored doors for a divider that can be rearranged according to the occupants' requirements. Look in second-hand office supply stores for portable cubical walls; these footed dividers are not only solid yet easy to move, they are covered with bulletin-board fabric and provide a modicum of sound deadening. For a more permanent solution, install accordion doors that can divide the room completely.