Snacks and non-alcoholic drinks are allowed. I bring them in. Sandwiches and other food, too. Yes, monorail transport to the parks is included if you are staying on-site. Even if you're not staying on site, it's either the monorail or boat from parking to Magic Kingdom anyway.
Sunscreen . . . well, doctors recommend it now for all seasons. But when you're going, it takes a while to get a "tan."
Actually, Monorail transport isn't just for on-site guests. Monorail goes between MK and EPCOT only, DHS/AK aren't on the Monorail. Monorail also goes to Poly, Contemporary and Grand Floridian resorts. There is bus or boat transportation for the other parks.
OP....my suggestion on picking the park for the last day.....wait until you've done the first round of parks and then choose your favorite. Which park is a favorite is an extremely individual thing....and with the age spread of your kids, I'm guessing it's going to be a bit of a debate for your family, lol. Each park has it's own personality plus of course the rides and attractions that you did on the first day that you MUST repeat again will help choose the last day's destination.
And yes, each park has lockers. MK has them inside the park under the train station, EPCOT they are to the ride of Spaceship Earth near the restrooms, DHS are next to Oscar's Super Service or outside by the Bus Info booth; AK by Guest Services or outside the park on the right of turnstiles near restrooms. Large lockers (17Hx12Wx22D) are $9 per day, Small (11H9Wx16D)$7. There is a $5 refundable key deposit. If you use a locker in one park and want to hop to another park, return your key to get your refund, keep your receipt and at the next park you can show your receipt and the locker rental is already paid....you do still have to give the $5 refundable deposit.
MK only has large lockers, and the other parks have other sizes, like luggage size etc.
Since you're staying off site, let me also mention that if you park your car at one park you can move to another park's lot without paying again, just show your receipt. Personally, when changing parks I prefer to go back to my car and move it to another park. The transportation between parks (except MK/EPCOT which has the Monorail), tends to be a bit slow...and remember you'll have to reverse it to get back to your car at the end of the night. To me, taking the time to hop with my car once means a quicker time getting home at the end of the night when I'm feeling the crash coming on, lol. I can go like an Energizer Bunny until someone tells me it's time to go home....and then the crash happens quick, so I want to get home quick, not have to do the cattle call to another park and then still walk out to my car, lol.
Enjoy your trip...that's a very very busy time to go, but quite exciting to be there for the New Year.
I am literally typing this at the airport gate waiting for them to call my flight to Orlando....I'll be in MK by this time tomorrow! YIPEE!!!