Looks like she is working on the outside pitch. So perhaps her shoulders have spun a little too open causing her to hit it off the end of the bat or at least not the sweet spot. This is exactly what my DD does. So either the tee needs to be a little deeper or her shoulders need to be less rotated at contact. Or a little of both.
The posterior chain is used for things like waiting on the outside pitch, creating direction during a swing as well as creating a ‘turn’ that is tight and efficient without much effort. Without it, getting the slack out will be compensatory.
I agree with your assessment of the issues the 2 young hitters face but for different reasons. One is directional, the other is functional. The posterior load or anchor has more than 1 duty in the overall scheme of things.
The posterior chain needs to be stretched out fully and maintained throughout for direction and functionality to be optimal.