@Pengggwn
A supermarket would be within its rights to ask you to leave or even prosecute you for this, because it is theft.
Point of order. In every company, every retail store, every supermarket I have ever worked in (and I have been in customer service/retail for eighteen plus years) the policy was actually no, you can't confront people for doing this because the reality is, until they are attempting to walk out the door with it without paying, it isn't theft, and "I haven't left yet and am intending to pay" is a valid defense to a shoplifting accusation.
We as staff are actually not allowed to confront these types of people until they try to leave the store with product that is not paid for. None of these posters saying they let their child eat from a barcoded item which they later pay for are shoplifting by this definition. Up to and until they attempt to leave the store without paying, they are still legitimate shoppers. Not shoplifters.
You can sermonize and moralize and clutch your pearls and "well I never" until the cows come home, but the fact is that legally, you are wrong.
And to address other PP, here's my two cents
- it IS only okay if the item has a barcode, because then it's going to scan at the same price no matter how much has been consumed
- it is NOT okay to let your child eat something that has to be weighed, because the portion that has been consumed is then inside your child and therefore cannot be weighed and paid for
- to the PP who mentioned hating scanning empty wrappers, perhaps working with the public is not for you if a little mayonnaise is enough to ruin your day. I have dealt with much worse, including being the only person in the store who was willing to clean up a puddle of pee. Mayonnaise? You have it good.
- ro the woman who had the projectile vomiting baby and busted open some wipes to clean it up, THANK YOU. I might have cleaned up pee, but vomit I can't do. Thank you for doing what you had to do and not making your child's bodily function some poor underpaid supermarket worker's problem!
(this is not sarcasm, I am genuinely grateful)
To every PP, including @Pengggwn, who has run mothers down for "pandering to tantrums" and "stealing" because they're "too lazy to be organized and bring snacks from home", you need to get a grip. A firm grip. Two hands, if possible. Because toddlers cannot be reasoned with. I actually agree that "your toddler is your problem", but I consider "feeding a child to avert a tantrum" to be a perfectly reasonable way of solving that problem, in a way that doesn't subject the rest of the store to a screaming tantrum for an extended period of time.
I wonder what the advice is, in lieu of feeding the child.
- Let them tantrum it out? (Bad parenting. It is making your child's tantrum impact on everyone around.)
- Reason with them? (Ridiculous. Toddlers cannot be reasoned with.)
- Leave the store and try again another time? (Also ridiculous. I don't know anyone who grocery shops for funsies. If you're there, with a toddler, the vast majority of the time it's not for recreation, it's because you need to be. Stopping and going home isn't always possible, or practicable, especially if it means taking the hungry, tantruming toddler home to an empty house!)
- Be more organized? (Not always possible. Life happens, and there's a whole lot of life happening all the damn time when you have a toddler. Nobody is perfect. Not even *@Pengggwn*.)
I also wonder how many of these self-confessed perfect parents who never, ever feed their child something they haven't paid for yet, would stand there tutting and making faces at the tantrums and the vomit, when these mothers who have found a perfectly reasonable solution to these problems decide no, we can't do that, it's stealing 
*In conclusion, there is a reason, in Les Misérables, that Jean Valjean is the sympathetic character and Javert is the unreasonable antagonist. It is because mindless, slavish adherence to the rules is not always just or fair, and what is just and fair is not always mindless, slavish adherence to the rules.