Training Bras

So my daughter is still very young. 5. She's asking a lot of "girly" questions...

She has 2 brothers who are 3 and potty training...and tend to still run around without underwear...

She has asked why she doesn't have a penis...

I told her that girls have boobs and a vagina instead...

She absolutely does not believe me because she doesn't have boobs...and yes I've told her they won't grow for a little bit yet...

She has questioned me about my bra and seems to identify those as "having boobs"...

Now I did start growing extremely early and FAST...I was a C before I left elementary school...and was a DDD by the end of 8th grade...I was almost a D before I got my period...

She does NOT have buds yet...so I'm not asking about function...

I'm thinking about getting her a plain cotton...no lace...no clasp...training bra...to help her understand being a girl...

Yes I tried the "Care and Keeping of You" book by American Girl...which seemed to cause more questions...I tried hair bows and dresses to help her understand being a girl without the big girl stuff...

She is growing up very fast...she's almost taller and weighs as much as my 8yr old...

She also asked why she doesn't have boobs like her uncle...

Opinions? Or ideas on how to help her understand...

Please keep the "let kids be kids" comments to yourself...I already want to cry at the thought that my baby girl is getting older...if she settled for simple explanation I wouldn't be posting...