Without reading through the responses (sorry, I might repeat some advice!), here is what I would do:
I would ask the parents/care-givers if they mind hand-me-downs (in a nice way, like Oh I have a load of stuff to get rid of and it might be Jane's style....), and either hand your daughter's clothes to her if they would fit, or actually go BUY some clothes at a resale/thrift shop and give them to her as the hand-me-downs you mentioned.
You could also just by-pass that entirely and if your daughter's clothes would fit her, you could tell her she can pick out a few things if she wants, saying "Oh we are going to have a yard sale soon, but if you want anything from this pile, be my guest." I swapped clothes with friends all the time at her age, so that should be fine and not seeming like "charity." You could even toss in a pretty bra or two that might fit, but there is not a guarantee you will get the size right.
Another option is to offer to have a "makeover party" for the girls and take them shopping, but buy only a few things, then you could even roll your eyes and say to the parents "Girls these days! They wanted to shop for NEON COLORED bras, of all things!!!"
If you know her birthday, your daughter could always give the gift of clothes too.
I wouldn't really mention anything to her. I developed early myself and have some pretty scarring memories of things other friends' parents (and my own!) as well as teachers said to me, and I was TRYING to cover them up/hide! :( It's a sensitive topic and a sensitive time in her life.
As a last resort, you could talk to a school counselor, as they might have better results really talking to the family about this.
Amanda