12 is an awkward age for that.
And how much of the fail blame goes where depends on whether this was a day trip to the ren faire or 'spend a weekend w/ grandparents' event--option 2 gives a kid a lot more chances to fail.
It's quite possible, at age 12, a parent is still packing his bag to stay at his grandparent's house...and if a parent forgot, the kid ends up staying for 3 days/the weekend SOL. (and, depending on the sizes and the family re
lationships, being at grandparents' house, realizing you don't have anything warm...well, there's not much to do about that. I'd have grabbed one of my grandpa's coats and he'd have been fine w/ that, but I could't wear my grandma's by that age)
I think my response would depend on whether or not there were other options. By 12, I was self-packing, but a "do you have something if it gets chilly' would have been a helpful reminder. (Heck, as an adult, it's possible to forget how fickle the weather is this time of year and I have 30ish autumns worth of weather in my memory. A 12 year old has about 6 autumns in memory

If the kid was reminded and declined to bring something, I'd let him suffer some consequences (B or C depending on how cold 'cold' was and what the souvenir plan/budget was).
If the kid genuinely forgot as a symptom of being 12...then it would depend a lot on the kid. (A or B depending on the kid--because there's a huge variation in temperament and irresponsible-ness and family re
lationships--if it were my kid, probably B. If it were my niece/nephew, probably A)