Activities to increase shoulder abduction (movement of the arm away from the body) include playing bird or airplane and turning a jump rope.
One way to teach this is to have your child do jumping jacks or jump rope while he recites the numbers.
There's so little material in a sling bikini you just might mistake it for a wadded up jump rope if you found one lying around.
"Wait," you may be saying, "What's cognitively challenging about jump rope?
She has double-jointed shoulders and can jump rope with her arms.