Get her to loosen up her arms and upper body and whip the bat. She has too much tension in upper body. Kids feel stronger when they create tension in arms but it’s having a negative impact on her swing. Have her focus on hitting with her legs (tension here) and letting the upper body relax. This will help her swing sequence.Would that contribute to swinging under the ball?
On contacting the bottom of the ball, it’s not a great teach but simply tell her to swing over the ball with a slight upper cut maintaining loose upper body