Yes, alert the mother, but in a non-accusatory fashion. A little humor can keep the situation from being overly embarrassing, and most parents will appreciate the opportunity to educate their child.

I'd tell her, but I'd whisper it in passing. If you get all accusatory and irate, then she's going to immediately get defensive to the point of lying about the situation. Bad example to set for the kid, who's embarrassed, and you're getting harassed when you don't need it.

So do it calmly, discreetly, and if she doesn't do anything immediately, walk away. You tried.

I would find a way to alert the mother without being rude or overbearing. A good way to go about it is to step up and very nicely say, "Excuse me... Hi, I'd like to try something different than my usual bag of Skittles. Would it be ok if I asked you child what their favorite kind of candy is?" And hopefully, that is well received and the child suggests a kind of candy (they'll most likely pick out what they are trying to take). Then, say something nonchalant like, "Thank you very much! May I buy you one too? Ok....Hey, let's not forget to pay for these! Do you have any other candy you'd like to get?" At which point, with any luck, the kid will reveal his stolen candy and you should give a glance to the kid's mother who is hopefully watching all of this unfold. If she is not, then get her attention and try to give her an idea of what's going on without sounding like you're telling on her child. She should then take over, but if she doesn't, then just simply and quietly tell the child: "You can't just take things from the store, it has to be paid for and always ask before taking anything, because taking things from the store without paying for them is stealing and people get into really big trouble for stealing. And you don't want to get into big trouble, do you? Just ask your mom what happens when people steal, she'll tell you. It's scary stuff." Then, wink at the kid or something and move on. Kids are pretty smart. They'd pick up on something like this really quick.

The point is to try to bring attention to what's going on and teach the child a lesson without causing too big of a scene. And with any luck, you can find a way to make sure the child's mother is watching everything carefully (give her a look or a motion or a nod or something to let her know that you want her to pay attention) so that she'll know when to step in and take over for the lesson giving. If she does pay attention and still doesn't understand or seem to realize what's going on, then you step in and finish it up as I described above (in a way that works best for you of course).

To be honest. No. Why? cause I don't want to embarrass the mom, but I do realize that the child may never learn.

If I saw a five-year-ol steal candy from a store, I would do everything in my power to alert the mother. It is important that he learns not to steal from his mother as soon as possible.
IF I could not alert the mother, I would take the initiative as an employee by calling the kid out on his theft. I would also explain that the mother missed her son's unacceptable behavior to placate her humiliation. In this way, the child would learn not to steal from both society and his mother.
Perspective Spanish Teacher

Of course, I have done it in the past and I am sure it will happen again in the future. I have just pointed out to the parent that their kid put candy in their pocket with a "just thought ya should know" tagged on the end. All little kids do it until they learn you have to pay for items. Sometimes parents let their kids pocket the candy until time to check out b/c it keeps them from screaming but they pay for it when they check out. I have found most parents just say "thanks" and go about their business.

I would basically and loud enough (In front of the mother) say to the child, "Don't forget to that you put that candy in your pocket. Mommy will want to pay for that."