lol funny aren't they? My DD was very similar....only wanted a sister.
We told her that our baby chose us especially because she'd make such a great big sister, and that mummy and daddy don't get to pick whether the baby that chooses us is a boy or a girl, we're just lucky to get such a special baby for our family.

Perhaps you could clue up the front desk people at your local Kmart (so that they aren't rude or abrupt) and take your DD in to show her that shops don't deal with babies?