honestly.. we don't worry about it.. there are usually many, many strollers at any "designated stroller parking" place and when we leave ours, we take out the things that are easily swiped from the basket (camera bag, purse, wallet, etc) and then just walk away.
now.. we have a LOT of stuff in our stroller.. several hundred dollars worth probably (we actually got a certificate for "most prepared for a two week safari" left on our stroller by a CM once.. guess that means we won the prize for the most loaded stroller!!! LOL!)... but you know what? If you can afford to get into Disney, then I think you can probably afford your own diaper bag, back pack, wipes, hand sanitizer, etc. etc. etc..
And besides.. if they need to steal AT DISNEY.. then heck, I figure, they must really need it more than me.. and karma will eventually level it all out.
I'm there to vacation, not stress... so, I make a point to be smart (grab my wallet and camera) and leave the rest to the Mouse and Karma!