I would guess Myers, David Jones or Pumpkin Patch, sorry i don't know many other shops but these usually have something nice (if not abit pricey but if it's a special occasion then why not occasionally ;-))
Myers has a great brand called Origami which is actually really reasonably priced for some of the pretty gorgeous dresses that they stock - average between $35-$50. Size wise I think they go up to around a size 8-10 so may not fit your older girl but you could still try Myer for a dress for her too.
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