Bring it, you may not need it. If you don't - not a problem. But they don't sell them at Disney (at least, not to the best of my knowledge), so if you don't have one and do need one, getting one may be a problem
We had ours leashed at 2 and 3 (my kids are 13 months apart), and did NEED it the first day. I have one darter and one daydreamer, and unless we have at least one on one defense going, its easy to lose kids. By day two, the kids knew to stay close, they'd figured out how easy it was to lose Mom (kids are short and everyone's legs look the same from down there - plus, kids are short, they can be standing five feet from you but in a crowd of adults, you can lose them.)
As someone said, watch the trip factor. And you might want to try it out before you go - some kids don't tolerate them. And don't use them as an excuse not to watch your kids. But most of us can't watch our kids every moment, and its those moments where your eye has been caught by a Mickey topiary or you are looking at the park map and your kid heads off another direction that the leash helps with.
And you probably want to take extra precautions with a toddler anyway (or any kid who can't rattle off Mom and Dad's full names and cell phone numbers) - tag them (a lot of people like tags from the pet store - we use a Sharpie) with a cell phone number - just in case.
We had ours leashed at 2 and 3 (my kids are 13 months apart), and did NEED it the first day. I have one darter and one daydreamer, and unless we have at least one on one defense going, its easy to lose kids. By day two, the kids knew to stay close, they'd figured out how easy it was to lose Mom (kids are short and everyone's legs look the same from down there - plus, kids are short, they can be standing five feet from you but in a crowd of adults, you can lose them.)
As someone said, watch the trip factor. And you might want to try it out before you go - some kids don't tolerate them. And don't use them as an excuse not to watch your kids. But most of us can't watch our kids every moment, and its those moments where your eye has been caught by a Mickey topiary or you are looking at the park map and your kid heads off another direction that the leash helps with.
And you probably want to take extra precautions with a toddler anyway (or any kid who can't rattle off Mom and Dad's full names and cell phone numbers) - tag them (a lot of people like tags from the pet store - we use a Sharpie) with a cell phone number - just in case.
We never used one, but I may have if I had seen those cutesie backpack ones. My kids never wanted to stay in strollers and there was always a lot of chasing going on
I was never one to force them to stay in a stroller - and I wonder why people think its OK to strap a toddler into a stroller to keep them contained, , but not strap them while they are toddling along to keep them nearby. If you think about it, it really is better to let little ones "out" to explore the world (and the World!) and if a leash helps you to keep them safe then I say go for it!
when I put it on her. She was panting, yipping, barking, it was not fun (although it is a little funny in hindsight).

We used a harness kind of like the English one posted, and if anyone frowned, I missed it 
I say go for it! I know I will be!