Alas, the consequences go well beyond losing half your marshmallows.
Earlier this week, I bought blackberries, which were requested so that they could be put into yogurt in the morning. Alas, this never happened, because the person in question was sufficiently short on time and impatient that the berries were consumed on their own before the morning came.
The quote came unprompted: "This is why I can't have nice things."
As a result, there will be no more berries, because if that's what's going to happen to them, they're not worth buying.
What happens when the child goes to interview to get into the best possible primary school, and they put two marshmallows in front of her? The payoff isn't just one versus two. It's one marshmallow versus two... and a leg up on her entire future. This will happen again when she interviews for a job, tries to make a friend, goes on a date, you name.
Once discipline becomes a signal, discipline becomes far more important than it ever was before, thus forcing us to engage in bigger and bigger feats of discipline, so we can send a costly signal that we have discipline, and suddenly that's eating all of our consumer surplus... from life, or at least childhood.