Each scout silently chooses one other scout that he will mimic.
Whenever that other scout moves, the scout should copy his movement, making it a tiny bit bigger.

If everyone remains frozen and not moving, ask all scouts with a [pet dog, older brother, birthday this month, ...] to slightly move any body part once.
That should prime the pump and get things going.

There is no 'winning', but the activity should build as the slight movements become magnified. Have an obivous signal, such as a whistle, to stop the commotion.