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 relationships
, 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 relationships
--if it were my kid, probably B. If it were my niece/nephew, probably A)