^ I just grabbed my AR-15 to see how comfortable this would be and there is no gap between the guard and the magwell. It would be impossible for her to get her fingers behind the magazine like that and even if she could she'd be jamming her thumb behind the trigger.
You can absolutely stick your thumb through the trigger gaurd but you're literally palming the entire magwell at that point, and also it would be very very bad if you ever forget to toggle the saftey.