First, I do think trying to do Disney with nothing wider than 50mm will prove to be an exercise in frustration. By the time you back up to get the shots you want, tons of people will get between you and your subject. And if you do character shots, it will be very hard to do anyhting but close-ups. If you do go with your current lenses, you will need to adjust your expectations for photography in the parks.
Second, I absolutely agree with the above recommendation. If money is really tight, then get another kit lens. Just be aware that there have been a number of kit lenses through the years on the Canon Rebel models. Some have gotten better reviews than others. Just be sure you're getting one of the more recent ones with better reveiws. I do know the ones with IS were more recent. I seem to recall the one that came with my XT or XTi (don't remember which) got lesser reviews.
If you want to move the ball forward a bit, then absolutely get the Tamron listed above. I did a lot of research on this class of lenses last year. The Tamron non IS is supposed to be a great lens and is very sharp. Many here on this board reported absolutely loving this lens. Do be aware Tamron came out with an version since then with Image Stabilization (I think they call it VC). But that version sacrificed some sharpness. And you'll pay more for it.
Good luck and let us know what you do!