Actually cup size gets larger as you size down in the band--for example, a 34B is roughly equivalent to a 32C.
If your daughters have IT tendencies, I'd concentrate on finding the right band size--something that fits snugly on the first set of hooks without feeling constricting is the way to go. You can get a rough idea by using a tape measure but there is so much variation between brands that you really need to try a bra on to get the right fit. If you fit the band on the first set of hooks, you will be able to tighten the band when gets stretched out after a few months of wear. One of the biggest fit errors for most women is wearing a too loose band because it feels more "comfortable". Actually, the opposite is true; a band that is too loose will be constantly shifting around when you move and won't offer any more support than wearing a T-shirt.
Once you've found the right band size for in brand you are fitting, then I'd look for a cup that fits without any spillage at the top and sides, or wrinkling in the cup once the straps have been adjusted to the correct length. The straps shouldn't carry any weight and, ideally, a bra should stay in place even if you slip the straps off the shoulders. I'd also eye-ball how your daughters' breasts sit on their chest. If the breasts are spaced out, I'd look for a bra design that has the straps set to the sides, while if the breast tissue tends more towards the front and center, I'd look for a bra that positions the straps more or less in the centre of the cup. You should also check to make sure the center gore of the bra sits nice and flush on the chest between the breasts--some designs will have quite a wide centre gore while others can be quite narrow.
I'd also not be overly influenced by the letter on the cup. Your daughters could look more or less the same from the front but one might be in a 38A while the other might fit nicely in a 32D. That doesn't necessarily mean daughter number two has a bigger bust because both cup sizes fit more or less the same breast volume. The difference is daughter number one has a broad back while daughter number two has a much narrower back.
I'm totally admiring you, Kelly, for helping your daughters understand how to correctly fit a bra right from the start. They will be so far ahead of all those young girls who smush (is that a word?) themselves into ill-fitting bras for years, always complaining about how uncomfortable they are in their bras.
