Once your daughter develops breasts, take her to get her first bra. Make sure it's fitted properly. Most large department stores offer a bra fitting service including Marks & Spencer and John Lewis.
If she is too shy to be fitted, you can fit her yourself by following this guide. To help avoid her shyness during the measuring process, try to take all measurements from the side of her body, and not looking directly at her.
Take the measurement (in inches) from just under her bust, right around her ribcage. Make sure the tape measure is not loose, but also not so tight that she can't breathe! She needs to be relaxed and not holding her breath. It should be resting smooth and straight across her back and where the crease of bust meets her ribcage. If she doesn't have a crease, then simply measure under her swollen breast area. The measurement you are taking here is her back measurement. If the measurement is an even number (eg 28") add four inches and if the measurement is an odd number (eg 27"), then add five inches. In this example, this would result in a back measurement of 32".
To find the cup size measure around her body again, on the fullest part of her breast. The difference between your original measurement and this one will determine her cup size. If there's less than 1 inch - AA cup, 1 inch - A cup, 2 inches - B cup, 3 inches - C cup, 4 inches - D cup, 5 inches - DD cup, 6 inches - E cup, 7 inches - F cup, etc... So for example, if her back size was 28in and her bust size is 30in, her final bra size will be a 32B.
In your opinion, are the clothing manufacturers encouraging our children to grow up too fast by making bra's for children as young as 7? When will you consider buying your daughter her first bra?