I dont know how small this girls hands are, but mine are not large for a mans by any means and my thumb reaches all the way around the grip of my px4 fs with the smallest backstrap. it is one of the smallest grips on an autopistol with that setup. long as her trigger finger reaches the pad or the first joint, shell be fine.
Just be very carefull not to scratch the gun swaping the backstraps, they are tough to change .