Bridges with feet up about knee height and hands close together. I don't worry about what their legs are doing so long as they are stretching their shoulders. This is a little confusing for them as the have to compete a bridge with straight legs in their floor routine which I think is unrealistic and not safe for many girls at their level.
'Cat stretch' on knees with bottom up and hands stretched in front, they then push their arm pits to the floor (also gets upper back so take care).
One arm pressed against the wall (horizontal) and turning body into the arm (can also point arm up the wall a little more to get a different stretch.
I have a gymnast who has very poor shoulder flex but it is actually her pectoral muscles, she has physio activities which she has adapted for use at gym. The one that has really helped is lying on the beam with a jacket under her shoulders and allowing her arms to hang in different directions. Two weeks of doing the activities has made a huge impact and she has picked up a few new skills because of it. I am annoyed with myself for not figuring this kid out. There are many muscles witch control the shoulders range of motion, so the standard stretches will not help every kid.
This
Workout Wednesday had an interesting stretch/strength activity for the handstand, most of my athletes are too young to be trusted with weights, but for older girls it could work well.
I sometimes stretch girls by getting them to lye down and placing one of their hands on top of the other and then lifting their arms while holding shoulders down (make sure you lift at a point that wont put stress on their elbows).
The foam roller is also very good (for all flexibility).
For the active flex, lying on the floor holding a stick shoulder width and lifting is another good activity
Simply lying on the floor with arms up by their ears and trying to push arm pits to the floor. If one of my athletes can not get their arms down they generally get extra shoulder flexibility activities to do.
I am also interested in more ideas for shoulder flexibility. What do you use Coach Simon?