All children need to spend time on their tummies.  The American Occuational Therapy Association shares in Tummy Time Tip Sheet the following information:

TUMMY TIME is an important activity for your baby’s development and is endorsed by the American Academy of Pediatrics (AAP). Because the AAP recommends that babies sleep on their backs for safety reasons, babies need enough supervised Tummy Time during the hours they are awake to strengthen head, neck, and upper body muscles. Tummy Time helps to build the strength and coordination needed for rolling over, crawling, reaching, and playing. Remember that all babies benefit from Tummy Time, including newborns.

http://www.aota.org/~/media/Corporate/Files/AboutOT/consumers/Youth/Tummy-Time-tip-sheet.pdf

Children with multiple disabilities, especially those with visual impairments, may not spend enough time on their tummies.  This impacts their development.  For this reason, Dr. Nielsen developed specific equipment, the Support Bench, to focus on development in prone.  The support bench encourages strengthening of the muscles of the head, neck and back while allowing complete movement of the arms, hands, legs and feet.  Below are some pictures of students at the Narbethong State Special School in Australia working in prone in a variety of learning environments.
 
The young boy in this photo is in a prone position using a Support Bench.  He has mylar paper under his feet and uses his hands to play with a variety of objects in a pan.
Boy in prone over a Support Bench with mylar paper at his feet and a metal pan with a variety of objects at his hands.
Here we see four children, all positioned in prone over Support Benches, exploring a ball pit together.
Four students in prone using Support Benches surround a ball pit.
The young boy in this photo hangs from a hammock swing and pushes his feet against an ESSEF Board while he plays with a Position Board. Try to use a firm surface until the chest/stomach area, to encourage extension of the back.
Young boy in prone over a hammock swing with ESSEF Board at his feet and a Position Board under his hands.
In this photo we see a young girl positioned in prone over a therapy ball. She is pulling a variety of small balls off the velcro strips on the Position Board.  This is a good activity when a support bench is unavailable, however as the support bench has a solid surface under the chest and abdomen, it will better help to promote development of back muscles, extending the back instead of causing flexion of the back.  The soft and flexible characteristics of the therapy ball promote a kyphotic position (or rounded position) of the back.  It is important to recognize these differences in equipment to recognize which equipment will better meet the long term needs of the child.  When possible, a child that cannot sit up independently would benefit from use of the support bench instead of using equipment that has a soft or cushioned base of support, as it will offer greater opportunity to work on strengthing appropirate muscles of the back, neck and head.
Young girl positioned prone over a therapy ball.
Here we see two young girls playing on scooter boards who have a Velcro Board and Elastic Board on each side of them that they are sharing.  This is a great activity but pay close attention to the position of the children.  Due to the width of the scooter boards, one arm can have the shoulder joint off the board to allow for free movement.  The other shoulder must rest on the scooter board - thus blocking the movement of the shoulder joint and preventing free movement of this arm.  The support bench would allow for the same activity - with free movement of both shoulder joints - thus allowing free movement of both arms.  When you notice these small differences you learn how to change or alter the environment to provide the best opportunity for the child to be active.       
Two girls are prone on scooter boards while they play with balls on a Position Board.
Collage of prone activities