Of course, if a trip to the store requires significant equipment, you can imagine what a day in a Disney Park requires. However, it's very easy to overpack or underpack, and neither is very pleasant. It's very important to be organized so you are sure you have all you need and a little bit of what you might need. I love to tell the story of when I accidentally left our diaper pad (along with diapers and wipes) in the car when we spent a day at SeaWorld with a five-month-old. We didn't mean to do it, but we did because we were not organized and simply didn't grab all the stuff we intended to bring into the parks with us. Without going into detail, let's just say that a diaper blow-out in a restaurant while the baby is eating her lunch in your lap is a mighty unpleasant experience... ESPECIALLY when one member of your party has to run all the way back to the car to get clean-up supplies.
So here is our strategy. It should be noted that we use strollers instead of carriers, so you may need to adopt this if you are opting to go without a stroller or with a stroller that does not have a basket for storage. We are also a two-parent family that stays together at all times. If you are a single parent or have more than one infant, there may be more need to adapt. As your children get older, the need to carry tons of stuff does decrease. Our plan is for when you have at least one member of your party that is not toilet-trained.
We take two bags into the parks. One is a medium-sized backpack (ours came from R.E.I.) and the other is a J.J. Cole Logic Bag. Neither bag is huge, but the reason we take two is because one stays in the stroller at all times with supplies that are less urgent, but still important. Both bags are carried in a backpack style, and this is important for loading and unloading onto buses and monorails. Because really, you can carry as much stuff as you'd like, but in the end, you have to be able to effectively use your method of transportation without ticking off everyone else in line. And even if you drive your own car, there is a good chance you will have to take a tram to your car, which means folding your stroller.
The reason I recommend the Logic Bag specifically is because of all its very nifty compartments coupled with its ergonomic design. It keeps you very organized and distributes the weight of what you are carrying evenly across your back. The water bottle holder even keeps you a little cool at first. (If you have an infant, you may choose to use this for bottles instead of water).
So what's in each bag? Glad you asked. The lists are below. Remember, the backpack stays in the stroller, even when we park the stroller to visit attractions. The Logic Bag comes with us into every venue.

- Prepacked snacks or snacks in small ziplock bags or small 1-cup Tupperware-type containers. (the number you take depends on how many kids and how long you will be in the parks... but we always have at least two).
- Change of clothes for Mom and Dad (for water rides or a particularly nasty diaper blow-out).
- Plastic bag to hold wet or dirty clothes
- Burp cloths if you have an infant
- Extra receiving blanket or two if you have an infant
- Formula and water if you have an infant and are not breastfeeding. I don't recommend bringing in pre-made formula, even in a small cooler. It's hot in Florida. I just don't trust it.
- Ponchos (in case of rain)
- Sippy cup (empty... you can fill it at the parks)
- Your favorite analgesic for aches and pains
- Sunscreen
- Bug repellent or wipes
- Bandages and antibacterial ointment for the unlikely boo-boo or likely blister
- Extra camera batteries, memory cards, and a tripod.

- Diapers (enough for one day - don't pack too many. Diapers are available at really high prices in the gift shops if you happen to run out. Ask at the counter. But generally, you won't run out.)
- Wipes in a travel container
- Hand Sanitizer (or antibacterial wipes if you don't feel comfortable with the ingredients of hand sanitizers)
- Extra Pacifier (if the child is young enough)
- Camera
- Change of clothes for child (specifically for food issues and diaper blow-outs. I recommend two shirts and one pair of shorts per child)
- Money
- Tickets
- Mobile phone
- Pen and small notepad to jot down memories/track expenses/note photographs
