Back in the Spring of 2002, we took our 3 oldest children to Walt Disney World. They were 6.5, 5, and 3 at the time. I tend to like to have lots of safety measures in place ahead of time so I can just relax and enjoy when we go out together. Here is what we did for that day to set my mind at ease.
The children were all dressed the same in yellow shirts, red shorts, and white hats with socks and shoes. This made it easy to keep an eye on them in the crowds at the park. They each had a "fanny" pack they were told they had to keep on all day. In the fanny pack was an index card covered with contact paper. On the card, I wrote my husband's name, his cell phone number, and the phone number of where we were staying. I instructed the children that if we got lost, they should find a Disney employee (they all wear name tags) and tell them their parents were lost. Then they should show them the card.
I also brought child safety leashes for the two younger ones. When we first arrived at the park, we rented a double stroller. I told the boys they either had to wear a leash or be in the stroller - their choice. The stroller was a great investment because by the end of the day, all three children were happy to be in the stroller! It's important to remember that all the strollers look alike, and that park employees will move your stroller while you are on a ride or on line. I bought yellow ribbon that had been curled and tied it to the handle of our stroller. This made finding the stroller much easier.
Another concern with spending the day at WDW is the effects of the sun. First, use sunscreen. We put it on when we arrived at the parking lot, then again every few hours in the park, especially after Splash Mountain. The other effect of the sun is dehydration. We brought in water bottles for each child and made sure they had a drink before and after each ride. We refilled them with the water fountains around the park. Remember that the effects of the sun are there whether you can see the sun or not, so be careful on cloudy days as well.
If possible, borrow a copy of the Unofficial Guide To Walt Disney World. It will describe the advantage of FastPasses, which we loved. It also is helpful in planning your itinerary - which rides to get to when, etc.