Worked out with my 7 year old daughter yesterday. Her plank was arched... Her squats didn't go low enough... Her sit-ups were too floppy... But guess what? She enjoyed it and she was sweating! ...and she wants to do it again!
I could have critiqued her every move and dampened her spirit. She would have been trying but her effort would have felt to be not good enough. You can definitely give her advice and help your child develop a new skill, but 80% of your focus should be on what they are doing right...then only 20% on things they can improve.
So instead of "Head up...put your feet down...get lower" It may sound like "Yes! Keep it up! ...that's exactly where your feet should be! ...you're staying with me! ...pull your shoulders back like this..." Over time their skills will improve by you focusing on the 20%.
Just remember... you may need to model the skill or physically help their body learn.. most kids (and adults) need more than just verbal directions to know what to do. Imagine putting together an IKEA piece only with VERBAL DIRECTIONS!