My DS is almost 8. At WDW, I took him in the ladies room if he had to go. I am sure the mens room would have been fine b/c I feel so comfortable at WDW ... however, my child's safety is my first and foremost concern.
I saw a lot of boys of various ages in the ladies room w/their mother/grandmother/aunt/sister. I would not hesitate to bring my son into the ladies room. There is nothing to be seen ... there are doors on all the stalls, so no embarrassment to anyone.
Maybe, someday, Disney will install family bathrooms. I think about Dads w/daughters. I mean, what do yo do if you're a Dad w/your younger daughter who may need help in the bathroom? Just an idea, although the line for those might be longer than some of the ride lines! Maybe they could offer fast pass for family bathrooms - hahaha!