As far as I can tell, that would be a criminal act.
Georgia law § 16-8-2 - Theft by taking says:
A person commits the offense of theft by taking when he unlawfully takes or, being in lawful possession thereof, unlawfully appropriates any property of another with the intention of depriving him of the property, regardless of the manner in which the property is taken or appropriated.
Georgia law § 16-7-21. Criminal trespass says:
A person commits the offense of criminal trespass when he or she intentionally damages any property of another without consent of that other person and the damage thereto is $500.00 or less or knowingly and maliciously interferes with the possession or use of the property of another person without consent of that person.
If you know the store is refusing to sell you a product, and you eat that product anyway, that is theft. If you don't eat the food but just open it, that's still criminal trespass. Even if you're leaving them money, you're still taking and/or damaging their property without their consent.
Additionally, if they told you to leave, and you refused and instead started opening food items, you might be guilty of trespassing in the more traditional sense:
A person commits the offense of criminal trespass when he or she knowingly and without authority... Remains upon the land or premises of another person... after receiving notice from the owner, rightful occupant, or, upon proper identification, an authorized representative of the owner or rightful occupant to depart.
As to whether it was legal to give the candy to your daughter before the disagreement and refusal of service, that may depend on what the standard practice is. It seems to me that in most clothing stores you're supposed to pay before consuming the product - this isn't a sit-down restaurant. But if there's nothing else going on, I think the average store would refrain from calling the police if the person did not try to hide the evidence (for example, by stuffing the empty box on a shelf) and if the merchandise was paid for before the person attempted to leave the store, regardless of whether it's technically illegal.