Your trip sounds like heaven to me! I took DS and his friends a number of times while he was still in school (he's in the work world now), and they were the BEST trips ever!
12 and 17 are old enough that they can be trusted w/minimal supervision (particularly if the "supervision" is hanging in the parks with them!)...no "bravery" required...just have a BLAST!!
(If one (or more) acts up (or you think he/she might), here's a trick I used when I chaperoned a group of seven 11-year-olds and it worked like magic from then forward...I told them "the rules"--go where you want (within defined boundaries--this trip was to a college campus for a school program event), but be back at the appointed meeting place at the appointed time...NO excuses (late buses, events that didn't end on time...NOTHING, whether your fault or not) will be acceptable to the "be there, on time" rule...if anyone was late--by even so much as 1 second (and we "sync'ed" watches)--they would have to spend at least an hour and maybe longer (depending on the severity of the lateness) wearing the "leash of shame"--one of those "baby leashes" that fits around the kid's wrist and I would hold the end)...Of course, no kid would risk that! And no one was late, ever! (They believed me when I told them I'd happily enforce the rule--told them I love them all and having one on a leash next to me would make me quite happy!)...I had a couple of red-faced kids a couple of times racing like mad to make the deadline (they did), but no one ever failed to show up, on time!)....
As for snacks & drinks: when I did this type of trip, I put them all on the dining plan and they pooled the snack credits and got in-villa food at the gift shop, using snack credits...since you're using TIW, you could just take them to the store (they have a pretty decent selection of munchies at the BWV store) and buy what they choose, using the TIW (they do take it there, or at least they did last time we were there, last year)...
Bringing some stuff along with you makes some sense, too...but like the other responders, I wouldn't mess w/going off-site to go grocery shopping...Garden Grocer and WeGoShop are just "too available" (and not that pricey)...remember you don't "have to" do the food order in advance...you can do it on site if the kids find they would prefer (and you're willing to provide) b'fast/snack items that aren't available in the store...
Last thought: We ALWAYS have a refrigerator-full of leftovers from meals...Disney meals (whether sit-down or quick-serve) are generously proportioned, and even with teenaged guys, there were leftovers...the kids loved snacking on those as the week wore on...so "stocking up" may not make a lot of sense, at least til you're a day or two into the trip...
Have a WONDERFUL time...I'm jealous (in the good sort of way)!