There was a bad incident we witnessed at Disney where a guy who just wasn't watching his feet (people who aren't parents never watch their feet, have you noticed?) walked in between a little girl on a harness and her mother (had to be less than a two foot distance between the kid/mom) and tripped on the leash.
It was really upsetting to us, that poor kid got whipped around like a ragdoll and smashed into his knees, and he ended up tripping and falling on her. He was swearing, the kid had a bloody nose and was screaming, it was bad.
Don't do it. Just hold their hands, you need to keep them pretty close at Disney, anyways. Not because someone will take them, just because at times there are *so* many people there. There are spots all over disney where you can let them burn off steam if they need to, without being leashed.
After 47 trips with them as young as fetues, there is always a hand holding-stroller option. Sometimes it won't be pleasant and they will scream bloody murder, but you won't be the only parents in Disney with industrially pissed off young 'uns that aren't getting to do what they want.