You can take a mini cooler in the park with you. So you could freeze juice boxes or water bottle if you have fridge/freezer and take that in the park. We used to leave ours in the cooler and never had any issues. Same with our back pack full of snacks. Just don't leave anything in there you don't want to part with.
If you can find a spiral bound unlined notebook pack it with some Crayola markers (the fat ones). Your kids could share that for autographs if they decide they want them. If not you can use it to play pictionary when you are waiting for rides, at dinner, on the bus. I found little ones at Target many years ago that had color paper in them and we still use them now (it's been like 5 years).
I always start my day at the one thing we really want to see so we don't miss it. In Magic Kingdom that is Fantasyland. Our day was always much better once we got those rides out fo the way and if we wanted to come back to them we could. Ending in Tomorrowland is fun (night time with the lights) but you'll want to send an adult to get Fast Passes for the bigger rides.
I take a picnic blanket with me, just put it in the bottom of the stroller so the kids don't have to sit on the plastic. I use it when it's time to find a spot for the parade. Much easier to send one person with the blanket to find a spot (Adventureland/Liberty Square is good place for this). Just put the blanket down and you've saved room for at least 4-5 kids easy. You could use a towel from the hotel as well. If it's cold you can use it for cover, if it's hot it keeps little butts from being burned by the pavement (but it shouldn't be that hot this time of year).
In Magic Kingdom at the Barber Shop on Main Street, it's $5 to get sparkles and color in your hair. They will even put girl's long hair in a bun, stripe the top and then sprinkle with glitter/confetti. For boys they spike their hair with gel, add color to the tips and a put a Mickey head on the back of their heads with color. What a great treat for the kids for a low price!!!! We make it treat for the day - if they are good the day before then they get that as their treat the next day. 3 strikes and there is no treat. Everyday I had a treat in mind. EPCOT - slushies at the Coca Cola tasting center, MK - Sweets shoppe or hair, Hollywood Studios - Carmel apple to eat while watching Beauty and the Beast, Animal Kingdom - ice cream during the parade.