We took our DD8 and DS4 to WDW last March so I might be able to give some advice. Regarding MK, first they should get there before opening and then head straight for Dumbo because the lines will get obscenely long later in the day. Then they need to utilize FP. Get one for Peter Pan and then go to Philharmagic. By then the FP should be up. Before going on PP have them get a FP for Snow White or Pooh. After riding Peter Pan go on Small World (nothing like walking around the park all day with that song in your head). If there is still time to kill before the other FP then do the carousel. From there they should head to Toontown, which opens at 10:00 and meet the characters. Then they could head over to Tomorrowland for Buzz (and/or Stitch-but IMHO I would skip Stitch; not worth the time). Using the railroad will cut down on some walking for the kids. It's no fun walking around with whining kids who are tired of walking. As for Epcot, I can't help you too much there as we passed on that park. Didn't think our kids would enjoy it. Although they do have Kidcot sections that your friend's kids may enjoy. My kids loved MGM and Star Tours and Muppets are both must sees for them there. As well as the backlot tour. We'll be doing AK for the first time in June, but from what I hear, they'll want to hit DinoLand, the safari, Tough to be a Bug & Kahli River Rapids. Again, their mother (and their height) will determine if they can go on EE. They may want to consider Donald's Breakfastasaurus in DinoLand. It's breakfast w/Donald (obviously), Mickey, Pluto & Goofy.
As for what to carry, we took a backpack with us and had snacks and juice boxes for the kids for when they were stuck in line somewhere. We also carried their sandals in there so they could put them on when going on a water ride because our kids don't like walking around in wet socks or sneakers. Also, each kid had their own disposable camera so they could take their own pictures.
I hope this helps. Everybody's tastes are different, so whatever interests them the most at each park, they should plan on hitting it as early in the morning as possible. I hope they have fun.