Pre-teens can be difficult, they develop at completely different times and it depends which age pre-teen you are thinking of.
10 year olds? Probably a small (very small) A or aa or AAA. Or they could only just be starting and need a training bra or sports bra, obviously she doesn’t need to but if her nipples are showing it could be helpful.
11 year olds are probably the most difficult to determine, absolutely no one (over exaggeration) is the same. Again probably an A but girls can change at a lightening rate and one day be aa and the next be C. If you are heavier you can expect bigger breasts to someone skinny, if you are tall the chances are someone shorter will have smaller ones to you, and exercise can (again) come into play, if you are a ballerina, you will be smaller (probably), if you do lots of serious sport you can expect them to be small but this doesn’t always apply.
12 year olds are usually an A to a B. Do not be surprised if your twelve year old girl seems to have big for her age, chances are if you look in her year group she’s not as big as others or maybe she is but she won’t be too far off, maybe it was just my high school but over ten girls didn’t wear any sort of bra and others had some that seemed huge to me (this was in year 8, it makes me sound like a stalker but I was just worried that I was small, it became clear to me that I wasn’t after we switched PE groups).
The point is, no one can determine what the normal size is, sure you could take every girl out, measure them and work out an average but you’d be sent to prison if you did that and the next day people would have changed, then the next generation would be different.