I totally understand your worry. Both our boys (now 9 and 5) are very independent (read: wanderers
and bolters). You know the kind - the ones you get a stroller with a 5-point harness for? In kindergarten all the other parents wished for their children to be independent and all I could think was "be careful what you wish for".
That said we've taken them several times to Disney and have never lost them. Well, not for more than a minute or two. (And yes, we DO watch our children.

) Here's how we did it:
We have several sets of matching t-shirts in bright colors. We tie dyed them ourselves and they are unique. Yeah, it's corney, but it works! This is our #1 best way to quickly spot our kids, and every adult around knows which grown-up the kid belongs to. Funny how adults won't get between a mama and her child, so long as they know which kid goes to which mama. I don't think they even know they do it.
Hint: If you decide to tie dye shirts - get a complete set in all the kid/youth sizes. And do extras for the future. Yours will last and last, but they'll grow out of theirs quickly.
Our #2 most important tip - avoid crowds. Avoid them, avoid them, avoid them. Use one of the park planners like Tour Guide Mike, or the Unofficial Guide (we've even used both together - yeah - sick) to help you figure out where and when to be.
Those two things by themselves have pretty much done it for us. But there's more you can do - mostly stuff you've already heard.
Of course, label them. Since this is a problem for us everywhere, they literally have pet tags with our numbers on them they can wear around their necks. Now that they are getting older they're thinking the silicon bracelets are pretty cool - but your's are too young for that.
Take a picture of what they are wearing in the morning. Unless you're wearing matching shirts - then you just say they look like me.
NOTE: If you rent a park stroller there is
no keeping a bolter on board. Honest. I've run my youngest down as he was exiting the stroller, while moving, to chase after something. (Of course, that had the side benefit of ending the unexpected exiting for the rest of the day.

) If you've got the 5-point harness kind of kid - you might think about bringing your own.
Also, practice. My kids know that when I say - "OK, I'm leaving now, I'll come back and pick you up next week." And I walk off - it's time to exit. Because, I
will walk off. Why do they believe that? Well....
Find some place safe. A store with only one exit for example. Give them the warning when they are engrossed with something and leave. Don't go far - hide out at the next aisle or something - but the kid needs to look up and not see mom. Let them stew on it for a bit - how long depends on the kid - thank goodness ours don't panic, they just deal with it - and then find them. Be very unhappy when you find them. It only took once.
Insterestingly, we only had to do this with the oldest one. His reaction is convincing enough for the youngest that he just takes it on faith because big brother does.
You may or may not need to go this far - it depends on your kids. And with twins - well, you may want to take them one at a time.
And for those out there who are about to flame me for doing this? Be glad your kids aren't this independent.
Oh, one more idea. Go with another family who's kids stick close to their parents. We did this when they were 7 and 3. Awesome trip! They barely wandered off at all - they stayed with their friends. Most relaxing - we didn't have to be as vigilant.