The correct answer is that, "they're too stoned to count"...

Source: I've worked at McDonalds, anyone with any ambition finds a new job immediately, so the only staff that hang around long enough to become managers are stoners.

Not sure that I buy it.

A kid trained to be super polite doesn’t have a mom telling her people who’ve done them no wrong are ‘stupid’.

I do, they train the kid to do that not to be embarrassed when near the other soccer moms, they think they are gods give to earth and can do no wrong.

Worker should've commented.

When ya need the money you keep your mouth shut. Sometimes when you don't you're choosing to be homeless.

Maybe so, but then there goes your plausible deniability.

"What I put one extra nugget in? Sorry store-owner, it was just an honest mistake, I'll be more careful in future."

"I put one extra on purpose, basically giving away free food / stealing from the business."

See the difference?

Like walk up with a smile acting like you know nothing and say "I gave your son an extra one since he was so polite" it'll make the kid realize his mom was wrong and make her feel like shit silently.

There's definitely something disappointing about a single unwed mother diminishing the minimum wage worker.