It depends when they hit puberty. If it is really early many kind of big.
32A or 34A would be average bra size for a 12 year old girl.
34B
AA
There is no average since each girl/woman is different and breast size is based on DNA.
There really isn't an "average" size bra for a ceratin age group. Most girls, however, are about a A- to a B-Cup by the time they are 13 years old. Still, it is normal for a girl to be smaller or bigger than that.
32A
32B
D
average would be 32B
She could get a sports bra and they are comfortable that's what I had on I am a ten year old girl and have to where cup bras size triple A
34A
Yes.