I think writing it on the magic band is a great idea!!
I "lost" my youngest (when she was 6) at EPCOT! It was so scary! She was straggling around the round pin store right behind Spaceship Earth and when I noticed and back tracked to find her, she was circling the store looking for me. So for 3 minutes we were "chasing" each other around the circular store, not seeing each other. She caught the eye of a cast member who stopped to engage her and check on her, and when we rounded the corner, there she was.
So After that the lesson added to our "Disney Rules" (Try at every bathroom we see, stay within arms length of an adult, no chocolate in your pockets) is "if you get lost. Stand right where you are, and mom will come find you."
I know its obnoxious. But I have an extra iphone that we just activated for her (she's 9 1/2 now) that will be able to keep her in contact with us if something goes wrong, and entertain her while we are waiting, and additionally keep her from asking what we have planned next. But I know that's unreasonable.