I'm not a parent, nor have I lost anyone. But I have been lost a few times myself when I was younger (not at Disney however). It is a terrifying thing, knowing you are supposed to be with someone, but now knowing where to go to find them. But knowing what to do when it happens can make a world of a difference. I agree with everyone who plants some form of ID on their child in addition to briefing them on "what ifs" each morning. After all... how clearly are you thinking when you're panicing? Now try it at age 4! That way, even if you child forgets how to reach you, a CM or helpful passerby can.
Another thing you should keep in mind, is that although yes, strangers can be bad... there are a lot more good guys out there than bad guys. I bet 98% of all Disney guests are there for the same reasons you are, and many of which are probably parents with the exact same worries. I'd be willing to bet that if a problem were to come up, if people could identify that something was wrong, that you would have all the help in the world.
The last time I was at Magic Kingdom, I was in line for Jungle Cruise with some friends I met at DWT.com. A young couple wheeled up behind us with their very little baby boy in a stroller. The little guy drops his toy, so i pick it up for him. I notice at this point that the mom has stepped away somewhere, and only the father remained with the child. The toy hits the ground again, and I once again reach back to pick it up. Now I realize that nobody is with the child. The line in front of me begins to move, and my party begins to board the boat. I immediately decided that I wasn't going anywhere until this child's parents came back, and another in my group does the same. Fortunately, mom and dad just stepped out of line for a minute to speak to a cast member, and never actually let the child out of their sight. But they came back in time for us to board the boat along with the rest of our party, feeling a little bit better about ourselves knowing we didn't just leave a kid by himself in the middle of the jungle (yes... pun intended).
So yes, when you're on vacation with your young children in a place as busy as WDW... DO worry. DO keep an extra set of eyes on them. and DONT let your parenting slip just because YOU are on vacation. Parenting is one job you never go home from. But at the same time, DONT worry so much that you make yourself sick over it. You and your family are there to have fun and enjoy yourselves. Inform your kids, ID your kids, watch your kids... but don't panic your kids. Be safe, but remember to have fun too.