No I would not allow a 9 year old to wear a G String since G Strings are more for 16 to 18 year old girls and above.
A G String is more of a sexual type of panties and it would be very inappropriate for a 9 year old to be wearing a G String.
A 9 year old is too young to be wearing G Strings or any other sexual outfits.
I would not let a 9 year old wear A G String until she at least reached 16 years of age.