I can't help you with arms other than just practicing the arm motions over and over for the muscle memory - standing and then transitioning to T-jumps and finally adding it to a real jump. As far as the chest goes... When she jumps, she should be rolling her hips forward to bring her legs up not sticking her butt out and bringing her chest down. When my CP was little, we practiced the rolling hips motion by me standing behind her and holding her around her chest/torso. Then, she would bring her legs up into the toe touch position. The only way to do this with someone behind you is to roll the hips. The crotch should be facing forward instead of down if she's doing it right and that naturally brings the feet up toward the shoulders. A drill that can help build the strength for this is to lay on her back on the floor and try to bring her legs up and kick the ground on either side of her shoulders. The motion is feet together, up, split as coming up (around waist level), try to touch ground at the top.
Edited to add... Here's a picture where you can see what I mean about the hips rolling forward (ignore the arm placements and broken wrists)