Disney protocol is good to know. If a child gets separated, they will want to find a CM. So as you get to disney and into each land, be sure to point out the CMs. Their costumes, where they stand, the stores, their nametags. The BEST person to get to is going to be a CM at a register. So if there's a store right nearby, or a food stand, that's a good person to go to. Eventually the child will be taken to the lost parents area.
There are some CMs who actually ahve phones AND who can use them (without getting fired) who might be able to call you. But don't *rely* on that happening. And don't rely on your cellphone working while there, or you hearing it!
When you realize the child is separated, send one parent back to the last spot you saw them. Contact a CM. Make sure you notice each day what your kids are wearing. BEST thing to do is to take a digital picture of the kids each morning, so when they ask for a description you just pull up that picture and can show them! That eliminates the panic if you can't remember immediately. It's just right there on your phone or camera.
Try to have both parents be calm. If not both, hopefully one can be calm. It helps nothing to panic. And know that the CMs are doing their job; they have done this time and time again, Disney has done this time and time again, and your child will be reunited with you.
Back to the child. If the child can't find a CM immediately, the experts* on kid safety say that statistically, if the child chooses who to go to, they will generally choose a good person. (kids have radar that helps them avoid creeps on their own, have you noticed that some kids just don't like some people?) Choosing a woman with children around is statistically the best, and easy to say to a kid. Find a mom.

Of course all of this is highly offensive to men. It's offensive to my husband. But statistics in this case don't lie. It goes from safEST to less and less safe, and moms are at the top. Poor hubbys. My husband can't even volunteer backstage at my son's dance recital, because ONLY men allowed backstage are employees of the Y. All the women are background checked, but they have the extra layer of not allowing dads. Oh well.
Anyway! So "find a cast member behind a register (this is what to look for in each land), or find a mom". Hopefully the mom will alert a cast member and won't go rogue and try to find you on her own. And from your end ifyou don't get a call, get in touch with a CM immediately, describe or show the child, let them do their thing, and just stay calm.
*Gavin deBecker...read The Gift of Fear. Excellent book.