I know, losing your child in such a crowded, big, unfamiliar place is terrifying. But please try to remain calm. I once encountered a woman who lost her child in the Magic Kingdom. She was hysterically crying and screaming. Guests and Cast Members were trying to help her out, but she just kept screaming at them. This is helping no one. Crying and screaming will not help you find your child. So take a deep breath. Cast Members are specially trained to handle this exact situation. They know exactly what to do and will help you out. You will find your child.
Find the Nearest CM
Go into the nearest gift shop and find a Cast Member. I suggest gift shops because usually gift shops have phones behind the registers. Cast Members can easily contact one another to faster track down your child.
Take a Picture
At the start of each day, take a picture of your children and what they are wearing. This way if you do lose your kids, you can easily pull up the picture on your phone and show the Cast Members. This will help them recognize your kids.
A lot of parents have a rule along the lines of "I have to be in your sight at all times". If a child runs ahead to the point they cannot see their parents anymore, they are too far away. If you both stay in each other's sights, you will be less likely to be separated.
Tips for Children:
**If your children are too young to understand this, they might be too young to take to Walt Disney World
Point out a CM
At the very beginning of your visit, take a minute to point out a Cast Member. Explain to your kids that Cast Members wear white name tags. Explain to your kids that if they ever get lost, they should find the nearest Cast Member with a white name tag. As stated before, Cast Members know how to handle this situation. There is a Lost Child station at every single park, including the water parks. It is a safe, secure place where lost children are watched until their parents pick them up. Cast Members here will calm down the kids and play games with them or provide coloring sheets.
Have your kids memorize your cell phone number! If they get lost, they can have a Cast Member call your cell phone. They can tell you exactly where you child is and let you know that they are safe. This is an important survival tip not just for Walt Disney World, but for any situation.
As a Cast Member, I have experienced a Lost Child situation twice. In both cases, the children were not very far away from their families. It only took a couple of minutes to reunite the child to their parents. In one situation, the child was the one who came up to me. She explained that she was lost, and she told me where she last saw her parents. She stayed right next to me while I tried to call my managers. And she wasn't more than 5 years old! I was very impressed with how quickly and maturely this little girl reacted. If you parent your children and properly prepare them for these kinds of situations, they will be fine!