Angie, I went to Disney World last year around the end of February. Had my 4 & 7 year old with me. I didn't have a Crohn's card, and I was a little worried about the bathroom situation. Like the others said, there are tons of bathrooms! I never had to wait, most of the restrooms in the restaurants are right inside the front doors, several times I used them when we didn't even have reservations.
I was really nervous, but the bathrooms were actually my smallest issue. Even in February, it gets really hot during the day in Orlando. My biggest problem was getting fatigued with all of the walking and getting overheated. I brought lots of bottled water and gatorade in the backpack, and tried to rest in the shade whenever I could. I wasn't my healthiest during the trip, but I wouldn't take it back for anything. I have pictures of my family smiling and having a good time, and we have wonderful memories that we still talk about.
With Crohn's we don't get a lot of "fun" time, constantly worrying about whether we should eat or not so we don't hurt later or have to go to the bathroom. If she's up to it, I say go and have fun!
PS - the staff at Disney is AMAZING! They are so customer oriented, especially when it comes to kids. If Izzy has any issues at all while there, they will bend over backwards to help her out. Have fun, I'm so jealous!!