I'm like Jordan's MOM, in that I'm even more concerned when I have to go to the bathroom. My DS recently turned 8 and I'm still very uncomfortable letting him go on his own or leaving him ouside the women's washroom while I go. It's especially difficult since I always have 5yo DD with me, and it takes longer to help her with her hands, etc.
If DS goes in on his own, after about 30 seconds I bellow "Don't forget to wash your hands" through the door, and he knows he has to answer. Actually, he wouldn't ever, EVER forget to wash his hands, but I need to hear his voice. Fortunately, most of the time DH is with us, so it isn't an issue.
I also wanted to repeat Jordan's MOM's comment about the fact that some of the washrooms at WDW have more than one exit. Make sure you orient yourselves as to which exit you're meeting at before anyone separates to go to the bathroom.