OK, so my DD has a bad habit of not getting good extension after hitting the ball. It looks to me like she is keeping her front elbow locked and this is causing what I call "looping" effect with her bat. Any suggestions would be appreciated.
In getting my DD to get her elbows in to turn the barrel she started doing much like your DD. We just started doing a 5 in succession quick front side toss with wiffle balls and a 3/4" dowel as the bat. Quick to the ball with full extension and the quickly back for the next one. She really likes doing it and we'll see if translates to a full swing soon. No time to cast or dumping the barrel. It's also really easy to know when the swing is around the body and not through the ball.
Driving the ball to right field like SBF said is also a good idea.
Reverse hand drill is not an extension drill. Originally posted as an extension drill but, not what it teaches. RHD is a good drill but, not for teaching extension. Extension is a result of proper/good swing.