is it normal for an 11 year old's arm to jiggle?

Are you parents in the circus ?

Flaccid muscles are floppy and will jiggle when the attached body part[s] move. Yes, that's normal.

ABNORMAL.

well, it depends on how much jiggle there is...when you straighten your arm out there is naturally some loose skin, or else you would be feeling pulls and pinches every time you bent your arm...there shouldn't be like an inch or more of skin dangling from the bottom of your arm when you straighten it, but skin is flexible and will "jiggle" when it is moved and stopped quickly