Posted by rfvv on Tuesday, May 22, 2012 at 8:05pm.
1. "on"
2 and 3 -- both are OK, depending on what is meant.
2 means they are jumping upward (from).
3 means they are jumping downward (on, as if from above).
or 3 means they are jumping up and down on the ground, not on a trampoline or anything like that.
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