So the daughter pushes 'C', Mom pushes 'D', swallows all 5 M&M's, and with a full mouth says "Let that be a lesson! You can't trust anybody!"

I have seen various variations of this story, some told firsthand. In every case I have concluded that they are just bad parents. They aren't clever. They aren't deep. They are incompetent and banal. Even if parents try as hard as they can to be fair, just and reliable they still fall short of that standard enough for children to be aware of that they can't be completely trusted. Moreover children are exposed to other children and other adults and so are able to learn to distinguish people they trust from people that they don't. Adding the parent to the untrusted list achieves little benefit.

I'd like to hear the follow up to this 'funny' story. Where the daughter updates on the untrustworthiness of the parent and the meaninglessness of her word. She then proceeds to completely ignore the mother's commands, preferences and even her threats. The mother destroyed a valuable resource (the ability to communicate via 'cheap' verbal signals) for the gain of a brief period of feeling smug superiority. The daughter (potentially) realises just how much additional freedom and power she has in practice when she feels no internal motivation to comply with her mother's verbal utterances.

(Bonus follow up has the daughter steal the mother's credit card and order 10kg of M&Ms online. Reply when she objects "Let that be a lesson! You can't trust anybody!")

I suppose the biggest lesson for the daughter to learn is just how significant the social and practical consequences of reckless defection in social relationships can be.