A. First off, in case you're worrying: Just because little Sticky Mitts boosted a pack of Wrigley's doesn't mean she's embarking on a life of crime. No doubt her predilection for chewy, fruit-flavored sweets outweighed the good sense you've taken great care to instill. Set her straight, and chances are she won't ever seek out the five-finger discount again.
How? Start by explaining to your child that she's committed a crime and that she has to make it right. March her straight back to the store and have her, gum in hand, apologize to the manager for stealing. Few things are more humiliating for a child than having to fess up to a stranger, and it'll reinforce the point that stealing is something that society as a whole, not just her mom, condemns.
Then have her give the gum back, or if she's opened it, make her pay for it with her own money (if she has any), even if she has to turn her piggy bank upside down to do so.
If she has no money, let her earn it with a few chores. Then toss the gum -- she absolutely shouldn't get to enjoy the fruits of her thievery.
When my mom found a jumbo pack of Trident that I lifted from the local convenience store when I was about 6, she set the gum out on the counter as a silent rebuke. And when, after a week, she figured I'd finally gotten the message (it seemed like ages to me!), she handed the pack to me and made me throw it in the trash. Thus ended my criminal career.