Weird. Sometimes Flash still just refuses to do little things for no apparent reason (I gave up on relying on frame labels ages ago). I’m sure I’ve had scripted/attached MovieClips with children that contain motion tweens, and no problems even with many levels of children.
Make sure the graphic symbol for the tween doesn’t have an instance name that would interfere with anything else, and that the graphic symbol doesn’t have any unusual settings in the library. Not that it would, normally. Or maybe it’s some strange mistake like you set the tween layer as a guide, hidden, or mask or something, and you left a static image of the graphic somewhere. These ‘how did that get that way’ mistakes are often what get me. Or try a classic tween?
It’s not really solving the problem, but maybe you’ll just have to bite the bullet and use Actionscript to simulate the motion tween.