Her panicking at the situation, coupled with her tiny body and unfamiliarity with the effectiveness of her power caused exhaustion when she depleted her mana pool.
I wonder what they’ll do next. She’s like a little sister or mascot to the unit, but she really wants to help, so maybe calling for a magic tutor? An apprenticeship perhaps? Given the title, I don’t foresee a time skip, so I do t think we’ll be seeing her fully grown until the end (or she uses some kind of magic).