Eyes moving is fine, head movement would also be allowed, if there were no new frames added to create the animation, meaning if you're working just with idle skeletal animation, a head bobbing is fine. Turning the whole head to the side etc would be allowed only as a variation on a single body part (i.e. no frames in between). It would also mean you can't have an extra object you can hide/show (hat, mask), you'd have to choose one or the other - one change of body part, or extra object.