The parks will be crowded if you go in mid-year. The problem isn't so much the number of people going but their preferences. There will be earlybirds and the types who HATE getting up before 1 pm. You should make everyone send in their preferred wake time (I am serious about this) and break up the group into the earlybirds and the grumpies. The earlybirds HAVE to be at the park no later than FIVE MINUTES BEFORE they open the gates. The grumpies can NOT show up after 1 or 5 p.m. Those are the worst times to attempt entering any of the parks. Why? Some parks have the late night fireworks or afternoon parades. Around 4-5 p.m., I saw TONS of people flooding into the parks for the night parades and fireworks shows. The size of the crowds was unbelievable.
Parents absolutely MUST decide whether to split up big brothers and sisters from the younger ones. They might, in the rush to get to the next attraction, lose them. I suggest keeping the younger ones with the parents so the others can have room to move around.
Request a schedule for ALL of the parks (Disney MGM, The Magic Kingdom, The Animal Kingdom, River Country and EPCOT) or pick one up at the resort. Every park has different open and close times. Make sure everyone knows the close times. If they get there early, the lines will be very short and, in some cases, they might get to visit the same attraction twice! It happened to me. I was able to ride Space Mountain twice in a row. Make everyone write down or know a place to meet for lunch and where to meet when leaving for the day.
Educate them about FastPass. It will be VITAL to their visit. Make sure they understand that it does NOT allow them to cut in line. It simply allows them to use a faster line reserved for FP holders only. The FP line is almost always five times faster than the normal line. Set a designated meeting place for lunch. Sadly, you will probably have to break everyone up because if you cannot find enough seating spaces, reservations might be the only way out. If on a budget, avoid any posh restaurants. All of them require reservations. Without reservations, the wait time will be very long. The seafood restaurant at the Living Seas requires reservations to avoid long waits. I waited so long that I gave up and went to the cafeteria next to The Land. Keep an eye on the teens because Disney does serve alchohol in some places. (the cafeteria in The Land had the beer bottles within easy reach of most kids' hands!

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But of course, alcohol is a no-no when you're severely dehydrated. Switch to lemonade or Gatorade if available.
Test Track, Space Mountain, Big Thunder Mountain RR and most kiddie attractions will be jampacked after noon so arriving early helps big time. Make sure anyone under age 16 has an emergency contact card. Understand that, unless visiting one section twice, you might not get to see all you want. There will have to be cuts and sacrifices. If you are taking all the kids to the Star Jets in Tomorrowland, the wait times can be very long because they can only fit about a dozen people onto the ride. Consider taking them to see Imagination instead. The Haunted Mansion is a no-no for those under age 10. I went and was not happy to see parents forcing very young kids to go with them. They were terrified and kept crying. It was pretty dumb of the parents to do that. Splash Mountain is a complex ride and has frequent mechanical breakdowns. (In Disney MGM, Honey I shrunk the audience actually scared a lot of kids pretty badly. It is NOT a cute ride.) Do not expect to always be able to see Splash Mountain. I went in late July and it was under repairs even though a lot of the guests were angry. When it's running, expect the line to be the longest of any attraction in the MK. In EPCOT, Test Track and Buzz Lightyear always have the longest lines. Rule of thumb: the higher the temperature, the longer the line to Splash Mountain. The later in the day, the longer the line to almost all rides/attractions!
Also, Carousel of Progress might be a troublesome ride. It broke down when I was in it! The Tomorrowland Transit Authority is perfect for all ages. It's also a great way to rest your feet and even takes you inside Space Mountain.
If anyone has a Disney Rewards Visa, NOW is the time to start racking up as many points as possible. You MUST send for them at least three weeks before you leave. It's a great program and saved me over $50.
A fanny pack or a very small backpack is a MUST. You will die without it. You must pack a large Ziploc bag or two so deliate stuff like cameras or phones don't get wet (especially at Splash Mountain or the Norwegian pavilion in World Showcase). A water bottle is VITAL. It can be scorching hot in mid-year, from June to late August. The temperatures will easily hit 100. DO NOT leave without a water bottle. Disney charges crazy prices on everything edible. You can simply refill the water bottle at a drinking fountain instead of worrying about losing that flimsy paper Coke cup. A small towel (4x12 inches) is very improtant for keeping your face cool. I have no clue whether those blue spray fans are worth the $20. I got mine free with my Dream Rewards Dollars but it saved my neck. When I went, EVERYONE was carrying one!
If you can make reservations for any place to eat, DO IT NOW. It isn't unusual for there to be a two-month waiting period for reservations. Sometimes, it stretches to three months!
Seriously consider leaving the parks mid-day to rest for an hour or so if the crowds are large or the weather is very hot. It helps. Just have them stamp your hand before you leave. Do NOT lose the ticket stub. You will need it to get back in. If nobody wants to leave but is obviously too exhausted, I would suggest Ellens Energy Adventure or The Land if the Tomorrowland Transit Authority is too crowded but it usually never is. Both attractions are slow moving and indoors so you can cool your heels and rest. BTW, Ellens Energy Adventure is funny and quite long so I would recommend it. It lasted a whopping 19 minutes!
Last but also important. Never buy new shoes just before entering. Wear a pair that is already broken in. You will be doing a LOT of walking. Expect to spend half or more of your time just walking to the attractions. For example, walking from Test Track to World Showcase in 90-degree weather is not for the faint of heart. I did this and was ready to die. Expect to get some serious mileage from your shoes. Finally, the shopaholics: save the shopping for last. It's too easy to accidentally fill that backpack. The last thing you want to be doing is running to one attraction with a heavy backpack because your Fastpass return time expires in two minutes.
Of course, I'm not trying to be an authority on the parks. I'm just giving you some tips from my previous experiences. Some tips will work and some work but it helps to know them. I hope this is of some help to you and good luck on your WDW trip.
