Go back and reread my post. I did not say "accept" disappointment. I said "handle" disappointment. No matter how we try disappoinments will come for our children. It's up to us to teach them coping skills.
In my DD11's young life she has had stomach flu on her birthday, had another birthday party rained on, and just this year, after months of work and preparation ,she woke up too sick to dance on the day of the last Nutcracker performance- missed the cast party too. In all of these cases we helped her see that sometimes things do not go as planned but it is up to us how they effect us. In the OP's case, I really think the reaction is a bigger problem than the original issue.
In our family an 8 year old crying over popcorn would not be acceptable and I would worry if I hadn't taught my children better coping skills by that age. My children at 2 or 3 might have cried but any older and they would have been annoyed, as DH and I would have, over waiting in line and then not being served but we would have all just moved on. I can't imagine that any of us would still be talking about it once we returned home from a funfilled vacation.
I have really tried to teach my children to "not sweat the small stuff" and to me popcorn is small stuff.