The problem with the alarm thing is that Disney is *loud*. There is simply no guarantee you'd hear the thing. I'm quite sure that I wouldn't, b/c I have a hearing loss for high pitched sounds like digital thermometers and some rings on my cellphone.
When DS was 2 we only did a one-day trip to DLR, but 2 days at SeaWorld (they sold those buy one get 2 days passes), and I wore DS in my Ergo child carrier (on my back) and also used a rental stroller some of the time. He's a slippery and fast child, and can twist his hand out of ANY grip in a heartbeat.
I know that some people don't like using "leashes", but I think they are a sign of love for a child that you know is sneaky. We put collars and leashes on our dogs because we LOVE them and want them to not get stolen or hit by cars. We can put harnesses and leads on our children that we KNOW have problems staying with us, for the same reasons.
My mom used harnesses with me and my brother on days that she took us (and our malamutes) to the beach. She didn't want any of us lost or harmed, so it made sense!
Alas, my son seems to have taken on the characteristics of the sled dogs I grew up with, b/c he runs to the end of the lead and SLAMS into it, requiring me to either let him fall onto his face (to save my shoulder) or to hold onto it for dear life while pleading with him to stand straight so that we don't look like enormous jerks. We never did feel comfy letting him fall, so ultimately we stopped using the harnesses (one a plain blue one, the other a "monkey backpack" with a tail), but man oh man I *wish* it had worked! When I was out and about with the harness, just like whenever I nursed in public, I was so focused on DS that I never noticed any nasty looks, if they were thrown my way, and no one ever talked to me about it.
Anyway, it's worth re-thinking if you wish to, b/c for a child that you know to be sneaky and slippery, it is definitely a viable option! (unless you ingested enough malamute fur as a child that your DNA was changed and your son is part sled dog, LOL)